I guess you could say that cats are very vain. They spend a lot of time trying to look good by cleaning, licking, and preening themselves. In the process they ingest quite a bit of hair that has to be eliminated from their systems. My cats go outside and eat grass, not because they like it, but because it makes them throw up these hairballs. Not only cats, but dogs will eat grass. Dogs eat it when they are sick, all with the idea of throwing up what's not supposed to be in their system. I have noticed that some people here, who never let their cats out, grow potted grass in their house. I never heard of that, but it's a good idea if you never let your cat out. I guess I'm the fortunate one living in the country and so far back from the road. The problem with my cats, is that after they eat grass, they come back into the house and then throw up those hairballs. Yucck! It never fails. I will get up in the morning without turning on the lights, and step in something wet, an upchucked hairball and then something. Yeeesh!