When you pour hot water on gelatin powder to make a dessert, the powder seems to disappear. But it doesn’t really. It’s still there, but you can’t see it.
What has happened is that the hot water breaks up the powder into tiny particles with a string-like shape. These particles are so tiny that you can’t see them, but they are floating in the water.
As each tiny string cools, it begins to act like a magnet. It can’t pick up nails or tacks, but it can pick up water, and that’s exactly what happens more and more drops of water cling to each tiny string until the entire dish gels.
When the gelatin is refrigerated, it hardens because these strings are holding the droplets of water tightly together. But if the gelatin is reheated, the water is released from each string and the entire mixture becomes a liquid again.