Stefan: There are a few mammals who have managed to find workarounds to the whole "one set of adult teeth" thing. And oddly enough, figuring out how to do that has also meant growing your teeth in a different direction, too.

Take elephants. All living species of elephants have gigantic molars that look more like car batteries than teeth. They're huge, rectangular blocks made up of thin sheets of enamel. And when the elephants use them to chow down, they really only chew on the front part of the tooth. As the plates of enamel at the front wear down, they fall off the molar and out of the elephant's mouth. They have about six or seven molars in each quadrant of their mouth over the course of their lives, and as the first one gets worn down, the next one in line moves forwards, like a conveyor belt.

This system means that elephants have a fresh grinding surface for their entire lives.

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