Hot water can hold far more dissolved particles than cool water. As the solution cools, those extra particles are left with nowhere to go — so they stack together into solid crystals. In this experiment, Borax dissolves in hot water and gets poured into a hollow eggshell. As the liquid cools, crystals form along the inside walls, turning the shell into something that looks like a miniature geode.

When water is supersaturated, it holds more dissolved Borax than it normally can. You dissolve Borax in hot water and pour it into a hollow eggshell — the eggshell acts as the rock cavity where crystals grow. As the solution cools, it can no longer hold all the Borax, so the extra Borax forms crystals along the inside walls. The result looks like a miniature geode with sparkling crystal points.