What is inside a rattlesnake's tail?

In rattlesnakes, hissing and rattling their tails both serve as warning signals. Rattles are segments of keratin that fit loosely inside one another at the end of the snake’s tail. These segments knock against each other to produce a buzzing sound when the snake holds its tail vertically and vibrates the rattle. Each time a rattlesnake sheds its skin it adds another segment to the rattle.