The Muscle That Powers Breathing
Just beneath the lungs sits a dome-shaped sheet of muscle called the diaphragm. It is the main muscle of breathing. When it tightens and moves downward, it pulls air into the lungs. When it relaxes and rises again, it pushes air out. You spend most of your life completely unaware of it doing this thousands of times a day. A hiccup happens when this normally quiet muscle suddenly twitches in a way you did not ask for.
What a Hiccup Actually Is
A hiccup is a sudden, involuntary contraction of the diaphragm. The muscle jerks downward without warning, pulling a quick rush of air into the lungs. Just as that air comes in, the vocal cords at the top of the throat snap shut sharply. That sudden closure of the airway over the inrushing air is what creates the familiar "hic" sound. The two events together — the diaphragm spasm and the snapping vocal cords — make a single hiccup.
What Sets Them Off
Hiccups can be triggered by a long list of small everyday things. Eating too fast or swallowing a lot of air, fizzy or carbonated drinks, sudden changes in stomach temperature from hot or very cold food, a quick burst of excitement or laughter, and even sometimes nothing obvious at all. In most people they last for a few minutes at most, then stop on their own as suddenly as they began. They are usually harmless, just an oddity of how the body's automatic systems sometimes briefly misfire.
Source
This article was written using information from Wikipedia.