As you inhale, the diaphragm flattens, increasing the capacity of your chest cavity and causing air to flow in to fill the partial vacuum that is created. Breath is expelled as your diaphragm relaxes and returns to its resting position. When hiccups strike, the sudden contraction of the diaphragm causes a sharp intake of air. Milliseconds later the space between the vocal chords called the glottis snaps shut. That delay between the quick rush of air caused by the tightening of the diaphragm and the closing of the glottis causes the characteristic hiccup croak.
Hiccups are a universal part of the human condition.
Most of us experienced them for the first time in utero , about six months before we were born. But while the mechanics of hiccups are fairly well understood, their purpose is something of a mystery. Because the glottis closes so quickly, interrupting respiration, researchers are fairly certain that hiccups aren't caused by the same centers in the brain that control breathing. Some researchers now conjecture that hiccups are a reaction more closely related to the vomit reflex, and are a way for the intestinal tract to expel unwanted air. That would explain why hiccups are sometimes brought on by drinking carbonated beverages or overeating.