Rattlesnakes are known for their relatively heavy bodies and diamond-shaped heads. They are considered to be the newest or most recently evolved snakes in the world. Rattlesnakes have either a rattle or a partial rattle made of interlocking rings, or segments of keratin, the same material our fingernails are made of. When vibrated, the rattle creates a hissing sound that warns off potential predators. It is an extremely effective and highly evolved predator-avoidance system. Another rattlesnake characteristic is the "pit" on each side of the head, which is a heat-sensitive organ for locating prey.