Why do rattlesnakes rattle?

Answer: Warning predators to stay away

Warning predators to stay away ✓ — Correct! The rattle is a warning. The tail ends in a string of hollow, interlocking keratin segments, and when the snake shakes it fifty or more times a second they clatter against each other in a buzz that can carry thirty metres. Warning a large animal off before it steps on the snake avoids a fight and saves venom, which is costly to make. A new segment is added at each shed, though segments break off, so the rattle does not give the snake's age. Young rattlesnakes have only a silent 'button' and cannot warn you; they are venomous, but the belief that they are more dangerous than adults is a myth, since an adult carries far more venom.

Digesting food while moving — Wrong. Rattling has nothing to do with digestion—snakes digest by staying still, not moving. The rattle is purely an acoustic warning signal to threats.

Shaking off old skin — Wrong. Rattling doesn't remove skin. It's acoustic warning—rapid tail shaking of keratin segments alerts threats before striking.