Why do we hiccup, and why can't we stop?

Have you ever been sitting quietly when suddenly — hic! — and then again, and again? The more you want them to stop, the harder they seem to come.

The sound does not come from the twitch. It comes from a door slamming shut. A muscle called the diaphragm sits across the bottom of your chest. It jerks on its own, and you take a tiny breath in. About 35 milliseconds later, the glottis — the gap between your vocal cords — snaps closed. The air gets cut off. That is the hic. So a hiccup is really a breath that gets grabbed halfway.

Its history is even stranger. A hiccup is not a random spasm; it is a complete reflex with a very steady rhythm. Babies hiccup before they are born, and the nerves that make hiccups work grow earlier than the nerves that make normal breathing work. Premature babies spend about 2.5% of their time hiccuping. Hiccups have been recorded only in mammals — cats, rats, rabbits, dogs and horses — and tadpoles use almost the same simple reflex to push water over their gills. Both are switched off by higher carbon dioxide in the blood, and both can be stopped by the same kind of medicine.

So there are two ideas. One says hiccups help a nursing baby push swallowed air out of its stomach, making room for more milk. The other says hiccups are a leftover from the gill-breathing of our amphibian ancestors. Both are still just ideas.

Try it at home: next time you hiccup, hold your breath and take slow sips of water — the holding is the important part. Holding your breath raises the carbon dioxide in your blood, and that is exactly what switches hiccups off. Then ask someone to count the seconds between each hic. You will find the gaps are surprisingly even, because this is a reflex, not random shaking. (Do not put a paper or plastic bag over your head. That is dangerous.)