Watchly Safety Review

Liquid Sand Hot Tub- Fluidized air bed

Mark Rober

9:09 · 148.4M views · November 28, 2017

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Minor FlagsReviewed June 2026

1Minor Flag

4No Issues

This video is safe and genuinely educational for most kids, combining a spectacular hands-on experiment with clear, well-explained physics concepts. The creator builds a fluidized sand hot tub from scratch and uses it as a springboard to teach buoyancy, equilibrium, and fluid dynamics in an engaging, accessible way — making it one of the stronger examples of science communication on YouTube.

Content Breakdown · 5 Categories

Parasocial & Emotional ManipulationNot present

The video contains a mild retention appeal ('if you promise to hang with me for the next two minutes') but this is a straightforward, non-manipulative request framed around delivering educational value, not emotional coercion.

See evidence (1)

“'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'”

Engagement Baiting & Manipulative CTAsNot present

The video contains a mild audience retention request ('don't click away if you think science is boring') but no guilt-tripping, superstition CTAs, fake giveaways, or coercive engagement tactics are used.

See evidence (1)

“'Wait, wait, wait, don't click away if you think science is boring'”

Playful & Fantasy ViolenceNot present

The velociraptor analogy ('I try to bring people in with a catchy thumbnail... and then when they least expect it' followed by velociraptor growl sounds) is a brief, clearly playful and humorous reference with no realistic violent framing.

See evidence (1)

“'my approach to science is similar to velociraptor hunting patterns... (velociraptor growls) - Clever girl. (velociraptor roars)'”

Hidden Advertising & Sponsored ContentA little

The video description mentions CrunchLabs (the creator's own company) with a promotional offer, but this is the creator's own product and is disclosed in the description rather than hidden within the video content itself. The transcript does not contain undisclosed sponsorship language.

See evidence (1)

“Description mentions: 'I started a company called CrunchLabs... check out the current promotion where you get 2 FREE boxes'”

Low-Quality, Extreme & Stunt ContentNot present

While the video has a stunt-adjacent premise (sitting in a hot tub of sand), it is primarily educational and engineering-focused with substantive physics explanation, not shock-value or disposable-item destruction content.

See evidence (1)

“Creator sits fully immersed in a hot-tub-sized fluidized sand bed as a demonstration”

What Parents Need to Know

Parents can feel confident letting kids watch this one. The video teaches real physics — fluidization, buoyancy forces, pressure gradients, and equilibrium — using vivid demonstrations and relatable analogies like tug-of-war and helium balloons. There is no foul language, violence, or inappropriate content. The creator does mention renting equipment and links to parts in the description, which is a mild commercial element, but it's framed around DIY replication rather than product promotion. The Jurassic Park velociraptor clip is used briefly as a humorous analogy and is very mild. Kids who watch this are likely to come away genuinely curious about science and physics.

Talk to Your Kids About

1Why do you think some objects float in the sand while others sink — what does that tell us about buoyancy?
2The creator said it took 25 failed attempts before the design worked. How do you think that felt, and why is it important not to give up when experimenting?
3Can you think of other places in everyday life where air or water pressure changes depending on how deep you go?
4If you could build your own science experiment at home, what would you want to test and why?

Protected Viewing

What's This Video About?

I don't like sand. It's coarse, and rough, and irritating, and it gets everywhere. Fluidized beds are used in industry for various reasons. They also happen to be really cool to play with and swim in. I made a liquid sand hot tub by adding air in just the right way. Here is the build info: https://www.dropbox.com/s/rrt27f71gqjg41v/LiquidSandBedBOM.pdf?dl=0 Here is a video by the King of Random with more details on making one for yourself: https://www.youtube.com/watch?v=yIfi3j8AKmE I started a company called CrunchLabs where we build a toy together and then I teach you all the jucy physics for how it works. So if you want to learn to think like an engineer and have really fun time doing it, check out the current promotion where you get 2 FREE boxes at https://crunchlabs.com You should also go subscribe to the CrunchLabs YouTube channel cause we’ve got some bangers in the pipeline- https://www.youtube.com/crunchlabs Thanks to these folks for providing some of the music in the video: Podington Bear - https://song.link/little_dipper Ponder - https://youtube.com/@Pondermusic Laura Shigihara - @supershigi Andrew Applepie - https://soundcloud.com/andrewapplepie Blue Wednesday - https://soundcloud.com/bluewednesday

Frequently Asked Questions

What age is this video appropriate for?

We recommend this video for children aged 7 and up. The science concepts are explained clearly enough for curious younger kids, but the depth of the physics discussion — buoyancy, pressure gradients, equilibrium — is most rewarding for school-age children who can follow along and ask questions.

Is there anything unsafe or inappropriate in this video?

No. The video contains no bad language, violence, sexual content, or dangerous challenges. The creator does briefly show a clip from Jurassic Park as a humorous metaphor, but it is very short and mild.

Is this video actually educational, or just a cool stunt?

It's genuinely both. The creator builds a real fluidized sand bed and then dedicates a substantial portion of the video to explaining the underlying physics, including buoyancy, force equilibrium, and pressure. It's a strong example of science communication that doesn't talk down to its audience.

Does the video encourage kids to try anything dangerous at home?

The creator does explain how to build a small version using PVC pipes and an air compressor, and links to parts in the description. The small-scale version is relatively low-risk, but parents should supervise any DIY attempts and note that the hot-tub-sized version used industrial equipment.

Is there any advertising or sponsored content in this video?

There is no explicit sponsorship in this video. The creator mentions renting a nitrogen tank and links to parts used, which is a minor commercial element, but it is presented in the context of helping viewers replicate the experiment rather than as paid promotion.

What will my child learn from watching this?

Kids will learn about fluidized beds and their industrial uses, the physics of buoyancy and how it applies to both liquids and gases, and the concept of force equilibrium. They'll also see a great model of scientific persistence — the creator mentions going through 25 failed prototypes before succeeding.