A hiccup is a spasm of the diaphragm, a sheet of muscle that moves to let us breath. Mammals have diaphragms, and so do many reptiles and amphibians. For us, and most other animals, hiccups don't have a known function, but for tadpoles, hiccups are quite important. At the point in their development where they have both gills and lungs, hiccuping lets them get water to their gills, without getting it into their lungs.