A dome under your lungs
The diaphragm is a thin, wide sheet of muscle shaped like a dome, or an upside-down bowl.
It sits under your lungs and heart, and above your stomach and liver. It's the floor of your chest and the roof of your belly.
It has a few openings, so your food tube and some big blood vessels can pass through on their way down.
Down to breathe in, up to breathe out
When the diaphragm squeezes, it flattens and moves down. That makes more room in your chest, so air rushes into your lungs.
When it relaxes, it springs back up into a dome, and the air flows out again.
Try it: lie on your back and put a stuffed animal on your belly. Breathe in slowly. Does the stuffed animal ride up? Your diaphragm moved down and pushed your belly out of the way.
Working while you sleep
You breathe about 20,000 times a day, and kids often breathe even faster than grown-ups. Your diaphragm does most of the work.
All night, your brain sends messages down a nerve to your diaphragm, so you keep breathing without thinking about it.
But you can take over too. You can take a giant breath, blow bubbles or hold your breath for a little while.
You can't hold your breath forever, though. Your brain notices you need fresh air and makes you breathe again.
Hiccups!
A hiccup is a sudden squeeze of your diaphragm all by itself. You suck in a quick gulp of air.
Right after, your vocal cords snap shut and stop the air. That's what makes the 'hic!' sound.
Eating too fast, fizzy drinks or getting very excited can start hiccups. Most go away on their own in a few minutes.
Lots of people have a favorite hiccup trick, but no trick works every time.
Talking, laughing and sneezing
Your diaphragm helps control the air you use to talk and sing. Singers practice using it to hold long notes.
When you laugh, cough or sneeze, your diaphragm and belly muscles make quick, strong moves to push air out.
Belly breathing to feel calm
Slow, deep breaths with your diaphragm can help you feel calm when you're upset or nervous.
Try it: breathe in slowly through your nose while you count to four, so your belly rises. Then breathe out slowly through your mouth, like you're blowing on a dandelion.
Do it three times. How do you feel?