I'm sitting in a hot tub filled to the brim with solid sand. This is one of the coolest things ever. If you take a tub of sand like this and then add air in just the right way, it basically becomes a liquefied soup. In science, this is known as a fluidized bed. It's used in industrial applications such as powder coat painting or in grain silos to make sure that the grain flows smoothly to the outlet. With the air on, the top surface is nearly frictionless. It's like an air hockey table. And then when you cut off the air, it freezes everything exactly where it's at. So today I'm going to show you how to make a simple version like this yourself. And then because I became curious what it would feel like to be fully immersed in something like this, we made a hot tub sized version. As you can see from this 60-year-old video from the Royal Institute of Science, this demo isn't exactly new. And I've wanted to build one for a long time, but you can find literally no information online on how to make one like that. So I started combing through a bunch of patent drawings for massive industrial-sized fluid beds, and I noticed that instead of one big inlet, they all had a bunch of small holes to deliver the air. So starting with that in mind, it still took us 25 failed versions before we arrived at this design. As you can see, it's just a bunch of half-inch PVC pipes arranged sort of in a ladder. But the key is to drill two 1-mm holes 90 degrees apart from each other, and then do a bunch of those spaced 25 mm apart. Then you glue all the pipes together and just make sure you orient the holes down, so that will keep the sand from getting in. And then you just seal off the container with some hot glue. And then fill your container with either fine sand, or we found glass beads worked really well. And you can power it with a small air compressor like this, or you could just rent a large nitrogen container like this for about 30 bucks, and that'll give you 10 minutes of run time. As usual, I will put a link to all of the parts I used to build in the video description. So my buddy Ken had an old broken spa in his backyard, which we decided to put to good use. Wow! It just stops and you're like locked in place. It's like a weighted blanket on every surface of your body. Now that it was tested and ready, in my ongoing quest to be the favorite uncle, I decided to surprise my nephews with it. Whoa! Yeah! Before we get to swimming in sand, let's talk about the science behind a fluidized bed. Wait, wait, don't click away if you think science is boring. If you're new here, you should know that my approach to science is similar to Velociraptor hunting patterns. I try to bring people in with a catchy thumbnail or a cool thing to see, and then when they least expect it... Clever girl. And then leave the analogy breaks down a little at the end. But if you promise to hang with me for the next two minutes, I promise to try and walk the delicate line between trying to make things simple to understand without oversimplifying or being condescending. Fluidization is just when you have enough upward air so that the force of the air pushing on each grain of sand is equal to the downward force of gravity or weight. And when that happens on any individual grain of sand, it hovers in equilibrium like this. You can think of forces acting on an object like a tug of war. And if an object is in equilibrium, that means there's a tie and it doesn't accelerate in either direction, just like in a real tug of war if the sides are even. However, if you add Stone Cold Steve Austin to this side, that will make it totally unbalanced and will start to move this way, which is actually what happens if you accidentally turn the air up too high. So fluidization occurs in that sweet spot where the sand particles hover at equilibrium, which makes them easy to move around. This is also the reason sometimes you'll see pools pumped with air to cushion the landing for the divers. Now you may notice that some stuff floats in the sand and other stuff sinks. This has to do with the buoyancy force, which is a function of the density of the surrounding fluid and the volume of the object itself. Whenever an object exists in a pressure gradient, there are forces from pressure pushing in on all sides, but they push a little bit harder the deeper that you are, which is why it hurts more to be the bottom guy in a dog pile or it hurts your ears more the deeper you dive in the pool. And this makes sense because the deeper you dive, the more water there is above you pushing down. And if you add up the size and direction of all those arrows, a bunch of stuff cancels out and you're left with one net force pointing upwards. That is the buoyancy force. And if your buoyancy force happens to be greater than your force from gravity, you float. And if your buoyancy force happens to be less than the force from gravity, you sink. Now we usually think of buoyancy with water, but you could think of things like helium balloons being buoyant in our atmosphere. So here's a trick question. Which of these has a higher buoyancy force? It's actually the rubber ball. Buoyancy force has nothing to do with the density of the object, just the volume. So since the rubber ball takes up more space, it has a higher buoyancy force. But, you might object, then why does the rubber ball sink and the balloon floats? Remember, it's a tug of war. The rubber ball sinks because the force from gravity on the thick rubber skin and the air inside is bigger than the buoyancy force. But for the balloon, even though it doesn't have as big of a buoyancy force compared to the ball, it still floats up because that buoyancy force is bigger than the weight arrow from the helium and the thin rubber shell. And the helium will keep rising like a ball floating up from deep in a lake, and then it will eventually hang out where the density of the atmosphere is roughly equal to the density of the helium in the balloon because that's where the tug of war becomes equal. We made it through. Now back to my nephews. I can't get it out. Get it. Got it. And everybody starts to move as soon as
A man sits cross-legged in the center of a hot tub that is completely filled with fine white sand instead of water.
"I'm sitting in a hot tub filled to the brim with solid sand."
Setting: backyard — overcast, natural daylight
People (1):
• sitting cross-legged, wearing black short-sleeve O'Neill rash guard and blue, yellow, and red swim trunks, short brown hair — smiling, speaking to camera
The man sinks rapidly into the sand as it begins to bubble and churn like boiling water. A large yellow rubber duck is also in the tub.
Setting: backyard — overcast, natural daylight
People (1):
• sinking into the sand, leaning back, wearing black short-sleeve O'Neill rash guard and blue, yellow, and red swim trunks, short brown hair — laughing with a wide open mouth, surprised
Text: "youtube/MarkRober"
The man stands behind a table with a clear plastic tub of sand, explaining the concept. He pours more white sand into the tub.
"This is one of the coolest things ever. If you take a tub of sand like this and then add air in just the right way,"
Setting: backyard patio — bright, natural daylight
People (1):
• standing behind a table, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — smiling, animated expression
A pair of hands pushes into the now-fluidized sand, which moves and ripples like thick liquid. Orange ping pong balls that were on top are now buried and then resurface.
"it basically becomes a liquefied soup."
Setting: backyard patio — bright, natural daylight
People (1):
• hands and forearms visible
Text: "youtube/MarkRober"
A pair of hands gathers the orange ping pong balls from under the surface of the liquid sand and lifts them out.
Setting: backyard patio — bright, natural daylight
People (1):
• arms submerged to the elbows, wearing gray hoodie sleeve visible
Text: "youtube/MarkRober"
A close-up shot shows a hand making a fist in the glittering white sand, which flows around it like water.
Setting: outdoors — bright, direct sunlight causing sparkles
People (1):
• hand visible
Text: "youtube/MarkRober"
The man stands in his backyard, explaining the scientific term for the phenomenon.
"In science, this is known as a fluidized bed. It's used in"
Setting: backyard patio — bright, natural daylight
People (1):
• standing behind a table, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — serious, explaining
A metal rack is dipped into a vat of yellow, fluidized powder, coating it evenly.
"industrial applications such as powder coat painting"
Setting: workshop or factory — functional, overhead lighting
People (1):
• hand and arm visible
Text: "POWDER COAT PAINTING"
An animated diagram shows how air is blown into a grain silo to fluidize the grain, allowing it to flow out smoothly from the bottom.
"or in grain silos to make sure that the grain flows smoothly to the outlet."
Setting: n/a (animation) — n/a (animation)
Text: "siperm"
A dozen yellow tennis balls sit on top of the solid sand. The air is turned on, and a pair of hands easily pushes them all under the surface of the now-liquid sand.
Setting: backyard patio — bright sunlight
People (1):
• hands and arms visible
Text: "youtube/MarkRober"
A hand pushes a small toy boat, which glides effortlessly across the surface of the bubbling sand.
"With the air on, the top surface is nearly frictionless. It's like an air hockey table."
Setting: backyard patio — bright sunlight
People (1):
• hand visible
Text: "youtube/MarkRober"
A yellow and blue volleyball is half-submerged in the sand. A hand turns a valve on the PVC pipe, cutting the air. The man then tries to push the ball down further, but it's stuck solid.
"And then when you cut off the air, it freezes everything exactly where it's at."
Setting: backyard patio — bright sunlight, creating shadows
People (1):
• leaning over the table, wearing gray hoodie and brown shorts
Text: "youtube/MarkRober"
A toy Nemo fish is placed on the bubbling sand. A hand gently pushes it, and it appears to 'swim' across the surface before the air is cut, freezing it in place.
Setting: backyard patio at dusk — bright, focused on the tub
People (1):
• hand visible
Text: "youtube/MarkRober"
A yellow softball is dropped into the fluidized sand, creating a dramatic splash-like eruption of sand that creates a small rainbow in the mist.
Setting: backyard patio — bright sunlight from the side
Text: "youtube/MarkRober"
The man stands in his backyard, explaining his motivation for building the large-scale version.
"And then because I became curious what it would feel like to be fully immersed in something like this, we made a hot tub sized version."
Setting: backyard patio — bright, natural daylight
People (1):
• standing behind a table, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — smiling, gesturing with hands
A black and white video from the 1960s shows a scientist demonstrating a smaller, circular fluidized bed. The video then cuts back to Mark Rober explaining his difficulty finding instructions.
"this demo isn't exactly new. And I've wanted to build one for a long time, but you can find literally no information online on how to make one like that."
Setting: lecture hall — stage lighting
People (1):
• standing next to the table, wearing dark suit and tie, white, balding hair — smiling
Text: "Ri"
A series of black and white patent drawings for industrial fluidized beds are shown on screen.
"So I started combing through a bunch of patent drawings for massive industrial-sized fluid beds,"
Setting: n/a — n/a
Text: "Fig. 1", "U.S. Patent Oct. 28, 1975"
Mark Rober stands in his backyard, explaining the key insight he gained from the patent drawings.
"they all had a bunch of small holes to deliver the air."
Setting: backyard patio — bright, natural daylight
People (1):
• standing, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — explaining, looking directly at camera
A rapid montage shows various failed attempts to create a fluidized bed: sand bubbling unevenly in a bucket, a metal piece not sinking, large bubbles forming in a wooden box.
"it still took us 25 failed versions before we arrived at this design."
Setting: workshop — workshop lighting
Mark Rober holds up the final, successful PVC pipe assembly that forms the base of the fluidized bed.
"As you can see, it's just a bunch of half-inch PVC pipes arranged sort of in a ladder."
Setting: backyard patio — bright, natural daylight
People (1):
• standing behind a table, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — smiling, looking at the assembly
A close-up shot shows a drill with a tiny bit drilling a small hole into a white PVC pipe held in a vise.
"But the key is to drill two 1-mm holes 90 degrees apart from each other,"
Setting: workshop — bright workshop lighting
A side view through the clear plastic tub shows the PVC pipe assembly at the bottom, with the small drilled holes facing downwards.
"and just make sure you orient the holes down, so that will keep the sand from getting in."
Setting: outdoors — natural light
A bucket of white, fine powder (glass beads) is poured into the clear plastic tub, filling it up.
"And then fill your container with either fine sand, or we found glass beads worked really well."
Setting: backyard patio — bright, natural daylight
A large, rusty blue nitrogen tank stands against a wall, connected by a hose.
"or you could just rent a large nitrogen container like this for about 30 bucks,"
Setting: outdoors, next to a house — natural daylight
Mark Rober stands in his backyard again, introducing the large-scale hot tub project.
"So my buddy Ken had an old broken spa in his backyard, which we decided to put to good use."
Setting: backyard patio — bright, natural daylight
People (1):
• standing, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — smiling, gesturing
A montage shows the process of building the hot tub version: a man in a garage with tools, drilling holes in the hot tub, cutting copper pipes with a miter saw, and soldering pipes with a torch.
Setting: garage and backyard — mixed natural and artificial
People (1):
• working, wearing gray t-shirt, short brown hair — focused on work
Three men carefully lower the completed copper pipe grid into the empty hot tub.
Setting: backyard — bright, natural daylight
People (3):
• leaning over the hot tub, wearing gray t-shirt, short brown hair — concentrating
• leaning over the hot tub, wearing maroon t-shirt, short brown hair — concentrating
• leaning over the hot tub, wearing gray t-shirt, short brown hair — concentrating
A man uses a utility knife to cut open a large paper bag labeled 'FELDSPAR '90', and white sand pours out into the hot tub.
Setting: backyard — bright, natural daylight
People (1):
• hand and arm visible
Mark Rober stands on the solid sand in the hot tub, then turns on the air. He sinks down into his waist in the bubbling sand, laughing.
Setting: backyard — bright, sunny day
People (1):
• standing, then sinking, wearing black O'Neill rash guard and multi-colored swim trunks, short brown hair — laughing, looking shocked and delighted
Text: "youtube/MarkRober"
The air is turned off, and the sand instantly solidifies. Mark is stuck up to his chest, smiling at the camera.
"It just stops and you're like locked in place."
Setting: backyard — bright, sunny day
People (1):
• stuck in the sand up to his chest, wearing black O'Neill rash guard, short brown hair — smiling
The air is turned back on, and Mark struggles to pull his legs free from the sand, laughing and grimacing with effort.
"Ah!"
Setting: backyard — bright, sunny day
People (1):
• trying to stand up, wearing black O'Neill rash guard, short brown hair — straining and laughing
A time-lapse shows various beach balls of different sizes and colors slowly emerging from the surface of the sand as it's fluidized.
Setting: backyard — late afternoon sun
Text: "youtube/MarkRober"
A hand pushes a basketball down into the liquid sand, then scoops sand away from it, showing how the sand flows back to fill the space.
Setting: backyard — bright, natural light
People (1):
• arm extended into the tub, wearing black t-shirt
Text: "youtube/MarkRober"
A point-of-view shot from a rake being dragged across the solid sand, creating perfect lines like a zen garden.
"Now that it was tested and ready, in my ongoing quest to be the favorite uncle,"
Setting: backyard — overcast daylight
Text: "youtube/MarkRober"
Three young boys approach the sand-filled hot tub, looking curious. One of them starts playing with small plastic toys in the sand.
"I decided to surprise my nephews with it."
Setting: backyard — overcast daylight
People (3):
• leaning against the tub, wearing blue t-shirt and black shorts, short brown hair — curious
• standing behind the tub, wearing dark t-shirt, short brown hair — curious
• standing, wearing black hoodie, short brown hair — curious
Text: "youtube/MarkRober"
The air is suddenly turned on. The sand starts to bubble, and the toys and a large rubber duck begin to rise to the surface, surprising the boys.
"Whoa!"
Setting: backyard — overcast daylight
People (3):
• leaning on the tub, wearing blue t-shirt and black shorts, short brown hair — mouth open in shock
• standing, wearing dark t-shirt, short brown hair — surprised
• standing, wearing black hoodie, short brown hair — smiling, surprised
Mark Rober suddenly emerges from under the large terracotta pot, yelling and surprising his nephews, who jump back in shock and then laugh.
"Yeah!"
Setting: backyard — overcast daylight
People (2):
• sitting up from under the sand, wearing black rash guard, short brown hair — yelling, wide-eyed
• jumping back, wearing blue t-shirt and black shorts, short brown hair — shocked, mouth open
Mark Rober, holding a young boy, pretends to drop him into the fluidized sand filled with tennis balls. The boy is laughing.
Setting: backyard — natural daylight
People (2):
• standing, holding a child, wearing black rash guard and multi-colored swim trunks, short brown hair — laughing
• being held horizontally and blue swim trunks, short brown hair — laughing hysterically
Text: "youtube/MarkRober"
Mark Rober stands in his workshop in front of a pegboard of tools, speaking directly to the camera.
"Wait, wait, don't click away if you think science is boring. If you're new here, you should know that my approach to science is similar to Velociraptor hunting patterns."
Setting: workshop — bright, even workshop lighting
People (1):
• standing, wearing gray hoodie with a lightbulb logo and dark gray pants, short brown hair — smiling, speaking earnestly
A clip from the movie Jurassic Park shows a hunter aiming a gun. A velociraptor's eye is visible in the bushes. The word 'SCIENCE' is superimposed over the raptor as it lunges.
Setting: jungle — dappled, dim light
People (1):
• aiming a rifle, wearing khaki vest, short, graying hair — intense, focused
Text: "MOVIECLIPS.com", "SCIENCE"
Mark Rober continues his explanation in the workshop, making a balancing gesture with his hands.
"I promise to try and walk the delicate line between trying to make things simple to understand without oversimplifying or being condescending."
Setting: workshop — bright, even workshop lighting
People (1):
• standing, wearing gray hoodie with a lightbulb logo, short brown hair — earnest expression
A ping pong ball levitates in the stream of air coming from a blue hairdryer held by Mark Rober.
Setting: workshop — bright, even lighting
People (1):
• hands visible, wearing gray hoodie
A group of action figures are set up in a tug-of-war, pulling on a string with a ping pong ball in the middle. The ball is stationary.
"And if an object is in equilibrium, that means there's a tie and it doesn't accelerate in either direction,"
Setting: workbench — bright workshop lighting
Text: "Gravity", "Air"
A 'Stone Cold Steve Austin' action figure is added to one side of the tug-of-war. His team then pulls the other team over, demonstrating an unbalanced force.
"However, if you add Stone Cold Steve Austin to this side, that will make it totally unbalanced and will start to move this way,"
Setting: workbench — bright workshop lighting
A diver performs a dive into a swimming pool. An overhead shot shows the large area of bubbles created in the water to soften the impact.
"This is also the reason sometimes you'll see pools pumped with air to cushion the landing for the divers."
Setting: olympic diving facility — bright, outdoor light
People (1):
• diving, mid-air and green swim trunks
A white board shows an equation made of sticky notes: Buoyancy Force = Fluid Density x Object Volume x Gravity.
"which is a function of the density of the surrounding fluid and the volume of the object itself."
Setting: workshop — bright, even lighting
Mark Rober explains pressure gradients in his workshop.
"which is why it hurts more to be the bottom guy in a dog pile or it hurts your ears more the deeper you dive in the pool."
Setting: workshop — bright, even workshop lighting
People (1):
• standing, wearing gray hoodie, short brown hair — explaining with a slight smile
An animated diagram shows a rubber duck floating (buoyancy > gravity) and a bowling ball sinking (buoyancy < gravity).
"And if your buoyancy force happens to be greater than your force from gravity, you float. And if your buoyancy force happens to be less than the force from gravity, you sink."
Setting: n/a — n/a
Mark Rober stands in front of a wooden fence, holding a large red rubber ball in one hand and a blue helium balloon in the other, posing a question to the audience.
"So here's a trick question. Which of these has a higher buoyancy force?"
Setting: backyard — natural daylight
People (1):
• standing, wearing gray hoodie, short brown hair — smiling, looking at camera
Mark Rober explains that the larger rubber ball has the higher buoyancy force because buoyancy depends on volume, not density.
"So since the rubber ball takes up more space, it has a higher buoyancy force."
Setting: backyard — natural daylight
People (1):
• standing, wearing gray hoodie, short brown hair — explaining
An animation shows the red rubber ball with a large downward arrow (gravity) and a smaller upward arrow (buoyancy), causing it to fall.
"Remember, it's a tug of war. The rubber ball sinks because the force from gravity on the thick rubber skin and the air inside is bigger than the buoyancy force."
Setting: backyard — natural daylight
An animation shows the blue balloon with a large upward arrow (buoyancy) and a smaller downward arrow (gravity), causing it to rise.
"it still floats up because that buoyancy force is bigger than the weight arrow from the helium and the thin rubber shell."
Setting: backyard — natural daylight
Mark Rober drops the red ball and lets go of the balloon, sighing in relief after the science explanation.
"We made it through. Now back to my nephews."
Setting: backyard — natural daylight
People (1):
• standing, wearing gray hoodie, short brown hair — relieved, smiling
Three shirtless boys sit in the fluidized sand, laughing as it bubbles around them. A large yellow rubber duck floats in the middle.
Setting: backyard — natural daylight
People (3):
• sitting and blue swim trunks, short brown hair — laughing hysterically
• sitting and blue swim trunks, short brown hair — laughing
• sitting and blue swim trunks, short brown hair — laughing
Text: "youtube/MarkRober"
Four people (one adult, three boys) hold heavy objects (bowling ball, cinder block, rock, brick) above their heads, then drop them into the fluidized sand. The objects sink quickly.
Setting: backyard — natural daylight
People (2):
• standing, wearing black rash guard and multi-colored swim trunks, short brown hair — smiling
• standing around the tub and blue swim trunks, short brown hair — excited
Text: "youtube/MarkRober"
A yellow tennis ball is thrown into the sand, and it shoots up into the air from a jet of sand, like a geyser.
Setting: backyard — natural daylight
People (2):
• standing, wearing black rash guard, short brown hair — arms raised in celebration, smiling
• standing, short brown hair — watching in amazement
Text: "youtube/MarkRober"
Three boys are kneeling in the solidified sand, trying to dig out a small object with their hands.
"And everybody starts to move"
Setting: backyard — natural daylight
People (1):
• kneeling and blue swim trunks, short brown hair — concentrating
Text: "youtube/MarkRober"
A boy wearing a snorkel and mask submerges his head into the bubbling sand, then quickly pops back up, covered in sand.
"as soon as"
Setting: backyard — natural daylight
People (1):
• submerging head, short brown hair — wearing snorkel and mask
Text: "youtube/MarkRober"
The air is off. Three boys are stuck in the solid sand. One boy is trying to dig out a bowling ball but can't move it.
"I can't get it out!"
Setting: backyard — natural daylight
People (1):
• kneeling/sitting in sand and blue swim trunks, short brown hair — laughing, straining
Text: "youtube/MarkRober"
The air is turned on, and the boys easily lift a large paving stone out of the now-liquid sand.
"Got it!"
Setting: backyard — natural daylight
People (2):
• leaning over, wearing black rash guard, short brown hair — smiling
• kneeling, short brown hair — laughing
Text: "youtube/MarkRober"
In slow motion, a boy is held upside down by Mark Rober over the hot tub. Sand pours off his body and legs as he is lowered towards the surface.
Setting: backyard — natural daylight
People (2):
• upside down, being held by the legs and blue swim trunks, short brown hair — laughing
• standing, wearing black rash guard, short brown hair — smiling
Text: "youtube/MarkRober"
The three boys are kneeling in the solidified sand. One of them is holding a bowling ball. They are all laughing.
"Cut it."
Setting: backyard — natural daylight
People (2):
• kneeling and blue swim trunks, short brown hair — laughing
• standing in the background, wearing black rash guard, short brown hair — smiling
Text: "youtube/MarkRober"
A slow-motion shot shows a barrel being launched out of the fluidized sand by a powerful burst of air, sending sand flying everywhere.
Setting: backyard — bright sunlight
Text: "youtube/MarkRober"
A black screen with white, typewriter-style text appears.
Setting: n/a — n/a
Text: "Please consider subscribing"
The fluidized sand in the hot tub is shown from a high angle. Small rocks are dropped in, and as they sink, they write out the word 'SUBSCRIBE!' in the sand.
Setting: backyard — overcast daylight