Cats, Grass, and Cat Grass - Why?

If you've ever noticed your cat or another cat munching on grass you might ask yourself why.  Why do cats eat grass?  They're obligate carnivores, and cannot survive a meatless diet, so why do they get the munchies for grass?  The fun answer is: no one really knows.  There's a few theories going around, but not a whole lot of research. They have the time and money to put ants on stilts but not to figure out why cats eat grass.  To be fair, it's probably more fun as a researcher to watch ants get all confused.  But that means we get to have fun speculating.  The running theories right now are that cats:

1. Use it to get nutrients they don't get otherwise
2. They use it as a vomit inducer to hack up persnickety hairballs
3. It's instinct, because cats used to eat all of their prey, including stomach contents
4. They just like it (I like this one best, because I think its funny)

For the first, well, there are cats that turn their nose up at grass, and cats often have completely balanced nutrition that's completely plant-less.  So that doesn't seem quite right.  The hairballs theory seems to have a bit more clout.  Cats can't break down the plant enzymes, so they'll eventually hack it back up (if its just a little grass in clear liquid, your cat is fine, no need to worry).  This would also bring up anything else stuck in their digestive tract, like hairballs.  It doesn't help that part of the reason cats have hairballs partially because we've bred them to have fine hair or abnormally long coats that more easily get stuck in the digestive tract.  The third theory is usually grouped with the first, because at first they didn't need it for hairballs, but instead all the other indigestible matter that passed through their system (the fur and bones of their prey, for instance).  Wildcats don't usually regurgitate material though, so its unclear.  Maybe there's a difference accounting for the physical size of the cat, but on the other hand, perhaps not.

As for the last option, cats clearly seem to like doing it, so there's no way to disprove this hypothesis outright.  Regardless, vets agree that eating grass is a harmless activity.  If you let your pet do it, though, its best to have cat grass you grow yourself.  There are many kits sold online and it's easy to grow.  If you grow your own you won't have to worry about pesky pesticide or other chemicals that might harm your pet if they ingest it with outdoor grass, and as an added bonus, it'll keep your curious feline from getting too familiar with the other houseplants.