The chameleon's tongue is built out of a muscular tube wrapped around a long, thin, tapering bone that lies in the floor of the animal's mouth. When the tongue muscles are triggered to fire, they squeeze tightly on the bone. This makes the whole tongue shoot out like an orange seed squeezed between your fingers.
Good aim is achieved because the spring is coiled around a forward-pointing spike.
The spring has a sticky end that allows the chameleon to capture a robot fly. Learn more about chameleons.