Crystallization
Crystallization is what happens when dissolved sugar or other substances slowly form solid shapes as water dries up.
Think of it this way
When sugar water sits in a shallow dish, the water slowly dries up into the air. The sugar stays behind and forms solid chunks on the dish. As more water leaves, the sugar pieces grow bigger. The final dish holds a layer of hard, dry crystals stuck to the surface.
Explaining crystallization by grade level
When you stir sugar into warm water, it seems to vanish. But hang a string in that water and wait a few days. Tiny bits of sugar stack up on the string in neat patterns. That is how rock candy grows from sugar water.