Speaking of the constantly-growing front teeth of rodents, someone mentioned that if they're not ground down they become useless. That's true, but there's much more than that!

If not ground down, the teeth can kill the animal. They grow in a curve, and if through some accident the incisors are allowed to grow they will actually re-curve and drive back through the head and skull! I've seen a photo of the skull of an animal that died this way.

Dave Oesterreich