Hiccups are muscle spasms in the diaphragm and throat, and it happens to us at the worst times, when we least expect it.
The large muscle at the bottom of your chest cavity sometimes spasms (as do most all of the muscles in the body, at one time or another). When this happens, air gets sucked in through the mouth.
The air never actually reaches the lungs because another spasm occurs in the muscles of the throat at the same time. Any number of remedies are suggested to stop hiccups.
Some of the more common ones include breathing into a paper bag, drinking a big glass of water (sometimes from the opposite side of the glassor is that just a practical joke?), and massaging your belly.
But basically the same principle works here as with other muscle spasms: If you relax, calm down, take your mind off of the hiccups, and get on with your activities, the diaphragm will stop spasming all on its own.
Having said that, though, here is the only cure for hiccups we know of that actually works time after time.
We call it the Mary Poppins Cure, after the song from the movie: take a “spoonful of sugar” and swallow it dry. Your hiccups will be gone in seconds.