Watch: Former Nasa engineer and his rubber ducky chill out in hot tub filled with 'liquid sand'

Mark Rober thinks "water-filled hot tubs are soooo 2016". And after watching his latest video on YouTube, so do the 5.7 million-plus people who viewed it. The former Nasa engineer, who shares quirky science-centric videos online, showcased the "fluidised bed effect" of air pressure on sand.

"I am sitting in a hot tub filled to the brim with solid sand. This is one of the coolest things ever," the California-based inventor said while sitting in the custom-made tub. "If you take a tub of sand like this and add air in just the right way, it basically becomes a liquified soup."

He explained that the "fluidised air bed effect" is used in industrial applications such as powder coat painting or in grain silos to make sure the grain flows smoothly to the outlet.

"With the air on, the surface is frictionless, like an air hockey table. And then when you cut off the air, it freezes everything exactly where it's at," he said in the video.

It took Rober 25 attempts to make the sand flow like a liquid and he also showed off some of his experiments conducted in smaller containers. Wanting to experience what it must feel like to be submerged in "liquid sand", the 37-year-old and his friends built a tub that could pump air rather than water. The effect is mesmerising as the sand moves around Rober and his rubber ducky creating ripples and waves.

However, once the air is turned off, the sand settles back down. "It's like a weighted blanket on every surface of your body," Rober said, demonstrating how he has to yank himself out of the tub with a lot of effort.