What has teeth but cannot bite?

This riddle plays on two senses of the same word. "Teeth" can mean either the biting teeth of a living creature, or the row of pointed, ridge-like parts found on many everyday objects — used for grooming, cutting, gripping, or meshing. "Bite" specifically means clamping down hard enough to cause injury. So the answer must have teeth in the physical, shape sense, while being completely incapable of the biting action.

A comb has a row of thin, blunt prongs whose only job is to glide through and separate hair — it has teeth in shape, but no clamping or cutting action at all, so it satisfies both halves of the riddle.

• Saw: its teeth are sharp cutting edges built specifically to cut into wood or metal, so it can cause real injury.

• Zipper: its small teeth interlock only to fasten fabric — at most they can catch or snag skin, a mechanical pinch rather than a deliberate biting action.

• Gear: its teeth mesh with another gear's teeth to transmit motion, and can pinch or crush anything caught between them.

Only the comb's teeth are purely functional and harmless, with no way to bite — making it the answer.