Stomata
Stomata are tiny holes on leaves that open and close to let air in and water out.
Think of it this way
A leaf works like a tray with many tiny sliding panels. Each panel covers a small hole in the tray. When the panels slide open, air flows in and water vapor escapes out. When the panels slide shut, the holes close and nothing passes through.
Explaining stomata by grade level
Leaves have tiny holes on the bottom. These holes let air come in. They also let water go out. You can see proof by checking both sides of a leaf.