Supersaturation
Supersaturation is when water holds more dissolved material than it normally can, so crystals form as it cools.
Think of it this way
Hot water holds more sugar than cold water does. Stir a lot of sugar into hot water and it all dissolves. As the water cools, it can no longer hold that much sugar. The extra sugar falls out and forms crystals on the bottom of the cup.
Explaining supersaturation by grade level
When you stir Borax powder into very hot water, it all disappears. But as the water cools down, it cannot hold that much anymore. The extra Borax has to go somewhere, so it stacks up into tiny, shiny crystals you can see and touch. The cooler the water gets, the more crystals appear.