|Cats develop hairballs when they groom themselves by licking.
Cat's tongues have rough surfaces made of backward slanting papillae. Much of the hair can't be dislodged, and the cat is unable to spit it out. And as a result, loose fur is swallowed. Majority of this hair passes all the way through the digestive tract, but if some hair stays in the stomach it can form a hairball. Eventually the cat will vomit to get rid of the hairball.