A hiccup is caused by a sudden spasm of the diaphragm muscle below the lungs.

The longer version

A hiccup starts with an involuntary contraction of the diaphragm, the dome-shaped sheet of muscle that sits under the lungs and drives breathing. The sudden inward pull draws air in quickly, and a moment later the vocal folds in the throat snap shut, producing the sharp sound.

The whole event is a reflex arc involving the diaphragm's nerve and the muscles of the larynx, and it usually stops on its own after a short run. The timing of the two halves is fixed by the reflex, which is why the sound is so consistent from one episode to the next. Irritation of the nerves supplying the chest and abdomen, or a distended stomach, is thought to set it off in many cases.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.