I guess the post about my rattlesnake's rattle got Bethany (Horse Girl) curious. She asked me "What's inside a rattlesnake's rattle?" You know what? I always wondered that, too. So I did some snooping around, looking for answers, and here's what I came up with. Hope it helps.
A rattlesnake baby is born with a little "pre-button" at the end of its tail. Then, when the snake sheds its skin (every few months), the old, dried-up skin gets caught on that button at the tip of its tail, and a segment stays behind. Each time the snake sheds, a new segment is added and the rattle gets longer and louder. The segments whacking against each other create the sound. You'd think you could tell how old the snake is by how many segments of old, dried-up skin are in the rattle, but sometimes they get brittle and drop off.
(Not that you'd WANT to know how old a rattlesnake is, anyway!)