A hiccup is a sudden tightening of the diaphragm (DYE-uh-framm), a dome of muscle between your stomach and lungs.
Normally, when you breathe in, the brain sends a message to the diaphragm to tighten smoothly. The diaphragm flattens, pulling down on the lungs to draw in air.
But the diaphragm or the nerve that sends it signals can get irritated by many things, including eating too much or too fast. The diaphragm tightens suddenly, sometimes repeatedly. This draws in air quickly until the opening between the vocal cords snaps shut, causing the "hie" sound.
Hiccups don't usually last long.