Rabbits are coprophagus, which means that they eat their own poop. Much of the digestive process of rabbits is carried out by bacteria in their gut. This happens too far along the digestive tract for these extra nutrients to be absorbed, so it is necessary for the rabbit to recycle its droppings a second time. If not allowed to do this, they can very quickly develop vitamin deficiencies and die.

This process is similar to when cows chew their cud. The difference is that rabbits are incapable of vomiting or bringing the food back up as a cow does, so the food has to pass through through their entire system twice.