Poop.
My 8-year-old just got a pet "panda bear" hamster. This is a domesticated black and white version of Mesocricetus auratus, the Syrian hamster, which originated in burrows in the plateau of Syria and Turkey, where they are considered a pest rather than pets. In North America, they are also bred for scientific experiments. I have discovered that besides being cute, they eat their own poop. Talk about the ultimate educational experience.
Technically speaking
Coprophagy is the proper term for eating poop, and it turns out that quite a number of animals partake. Still, I do not recommend coprophagy for dinner or even dinner time conversation.
Warning: you may find this post in poor taste.
Down the chute
When Hammy nibbles on a piece of carrot, the carrot bits travel down to the stomach and into the small intestine, which absorbs what nutrients it can.
Help from Little Friends
The challenge of eating plant material is digesting cellulose, the stuff that makes plants tough. Most plant eaters, like cows, pandas, and hamsters, have some kind digestive system that relies on microbes to break down the long molecular chains of cellulose into sugars that can be used for energy.
Caecum, and you shall find bacteria
Hammy, like many rodents, has bacteria in a pouch in its gut, called a caecum, between the small and large intestines. This is related to your appendix, unless you've had yours removed because of appendicitis.
End Result
The cellulose heads into the caecum, where the bacteria break it down. The result passes into the large intestine, which is not built for absorbing many nutrients, so out comes soft poop.
One More Time
Hammy apparently finds this outcome tasty. It eats that poop and this time, more nutrients can be absorbed by the small intestine. The secondary poop is apparently harder.
Snooping on Poop
I have not actually witnessed Hammy eating its own poop. It can curl up into a ball and could be eating them just as they come out. Hammy seems to stockpile hard little poops that look like the bran nugget cereal I have for breakfast, but these might be the ones it doesn't want any more. More research is required.