How do rabbit have fur balls?

Rabbits groom themselves by licking their fur, just like cats do. The fur can collect in the stomach into a mass called a "fur ball" (or "hair ball"). When cats get a fur ball, they vomit it up so that it can't harm them; rabbits, however, can't vomit, so when they get a fur ball it can get stuck in their digestive tract, leading to severe illness (GI stasis).

The risk of fur balls is one reason why it's very important to make sure your rabbit is well-hydrated and eating a high-fibre diet.

Hydration: Rabbits should have access to cool, fresh water at all times. Water should be replaced daily, and the container (bowl or bottle) should be washed so that bacteria doesn't develop. You can also serve the rabbit's greens sopping wet.

High-fibre diet: The best way to ensure a high-fibre diet is for your rabbit to eat lots of grass hay. You should make fresh grass hay available at all times. Put some in the litter box, because rabbits like to graze and defecate at the same time. A few small piles of hay spread around their habitat is more enticing to rabbits than one big pile. You can also put hay in toys, like inside the cardboard roll inside toilet paper and paper towels. If your rabbit doesn't eat much hay you'll have to experiment with ways to encourage your rabbit to eat more hay. You should also choose a high-fibre pellet (18% minimum - see the nutritional information on the label). Other things that add to fibre content are cardboard and paper products, and twigs and branches - but make sure you're choosing the right kinds, because some varieties are toxic. Apple and willow are good choices.