Teeth a and b on the diagram show the herbivore's teeth. Carnivores have teeth which are shaped to slice and rip the meat they eat. Teeth c and d on the diagram show the carnivore's teeth.

As one gear turns, the other gear must also turn. Where the gears meet, the teeth must both move in the same direction. In the diagram, the teeth of both gears move upwards. This means that the ...