Put one hand on your chest and take a deep breath . . . hold it! Now let the air out. Do you know why your chest seems to get smaller? It's called exhaling. This means you're pushing the air you breathed in out of your body. Your lungs are kind of like balloons, and when you breathe in (inhale), they get bigger. Then when you exhale and let that air back out, they get smaller. That's why your chest goes up and down when you breathe.