As you hear a punch line, your brain ignites its centers for higher thought, muscle function, and emotion. Their conclusion: What a riot! The muscles around your eyes and cheeks start spontaneously contracting.
Your noggin shoots a signal to your brain stem, the body's HQ for lung functions like breathing and laughter.
Your diaphragm and chest muscles tighten, forcing air out of your lungs.
That air rushes through your windpipe, blowing over your larynx. Your vocal cords vibrate and emit short, unfettered vowel sounds like ha-ha or ho-ho.
If what has you going is really funny, your eyes start to water.
The sudden exit of air from your lungs creates an urgent call for oxygen. Your heart rate and blood pressure ramp up to help ferry more O2 to your organs.
While facial and core muscles tense, the rest of your muscles become weaker or less coordinated. Hence, it can feel impossible to walk straight while laughing hard.
Your obliques are also working to help expel air. You may burn a few extra calories.
A side-splitting laugh can help release endorphins, those natural opiates often triggered by exercise. Your pain threshold might shoot up, at least temporarily.
Loads of chuckling may also dial down production of the stress hormone cortisol—a happy thing, since too much cortisol has been linked to exhaustion and depression.
It's true: Laughter is contagious. Some scientists speculate it evolved as an early bonding mechanism. If they're right, sharing a laugh with someone could help the two of you connect emotionally.