Most of the time you breathe in and out and don't give it a thought. But if you've ever gotten the wind knocked out of you, you know that breathing can get interrupted. Getting hit in the belly or falling hard on your back can make you feel like you can't breathe normally for a little bit.

It's called getting the wind knocked out of you, but it turns out that it's not the air (or the wind) that's the problem. It's your diaphragm (say: DY-eh-fram). The diaphragm is a dome-shaped muscle under your lungs. When you inhale, the diaphragm pulls down to help pull air into the lungs. When you exhale, the diaphragm pushes up to help push air out of the lungs.

Getting hit in the stomach or back can cause a spasm in your diaphragm. That means this muscle contracts — or gets tense — instead of doing its usual thing to help you breathe. If this happens to you, take some deep breaths. You should feel better in a few minutes. If not, it's a good idea to get checked out by a doctor.