Rehydration
Rehydration is what happens when dried food soaks up water and returns to its soft, full size.
Think of it this way
A dried mushroom sits in a bowl of water. It starts small, hard, and flat. As it soaks, water fills the spaces inside and the mushroom grows soft and round. After an hour, the bowl holds a full, plump mushroom again.
Explaining rehydration by grade level
Think about hard, dry pasta. When you put it in water, it drinks up the liquid. The pasta gets soft and big again. Water fills the tiny spaces inside that dried out before.