recently, a short video is very popular, in which we can see a jar of bubbles, like fine sand of liquid. The lightweight ball floats in it, stirring it with your hand, and it feels very good.

(recorder: Rocky Cheng)

this effect can be achieved by filling compressed air at the bottom of fine sand, a phenomenon known as "fluidization". When the upward airflow between sand particles is strong enough, it can overcome gravity to lift sand particles and prevent them from gathering together. In other words, the fine sand particles are now evenly dispersed in the updraft, so that its overall performance looks like a liquid, with low-density objects floating on its surface and heavy objects sinking.

here is a more magical demo from Professor Martin Rhodes. The duckling originally buried under the sand suddenly surfaced with the fluidization:

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this is not just for fun. The fluidization state of solid particles provides a very uniform mixing, and the contact between the two phases is also very sufficient, so it has many practical applications in industry and other fields.

but I think the most important thing you want to do is to put your hand in and stir it.