Mark Rober is an ex-NASA JPL engineer. He designs elaborate experiments that make learning about science truly fun. His latest experiment is a sand-filled hot tub in which you can swim. How can you swim in the sand? We saw how sand acts when air is passed through it.
The sand-filled hot tub works on the same fluidization effect which allows it to act like a liquid when air is passed through it. Air travels upwards from pipes situated at the bottom of the hot tub and travels between the granules of the sand and reduces the friction between them, causing the sand to behave like water. Lighter objects float while heavier objects sink to the bottom as long as the air is flowing. As soon as the air supply is cut off, the sand particles lock into place and trap everything where it is.
You can watch Mark Rober enjoying his sand-filled hot tub in the video below. It is very amazing.