A hiccup is a contraction of your diaphragm, the muscular membrane in your chest that figures importantly in breathing. Inhaling contracts the diaphragm and exhaling relaxes it.
Hiccups are spasmodic, involuntary, and as useless as they are annoying. A post-Thanksgiving distended stomach can irritate the diaphragm and cause a fitful spell of hiccups. Exercise or stress can cause them as well. More often, though, the reflex has no apparent cause.
One hypothesis suggests that hiccups may be a remnant of a primitive sucking reflex. Whatever the ancient function, they are little more than a nuisance now. And everyone swears they've got the surefire cure, whether it's holding your breath, giving yourself a good scare, sucking on grains of sugar, drinking water upside down or even (one reputable journal reports) a gentle, modified Heimlich maneuver. Many of these actually work, by temporary halting the rhythm of respiration.