My three year old son is walking around the house conjugating verbs. Of course, he's doing it in the way my children always do things, so he just walked by chanting to himself:

I have a butt.
He has a butt.
She has a butt.
You have a butt....

This reminds me of how my older children first learned how to distinguish a noun from any other part of speech. As we all know, a noun is a "person, place or thing." The three of them (they were 6, 4, and 4 at the time and desperate to play Mad Libs) couldn't remember that, so they invented the definition, "A noun is anything you can poop on." And it works! You can poop on Grandma, you can poop on Saturn, and you can poop on the car, etc. etc. They've never had trouble with it since, although I suspect that they might have some trouble when they get to gerunds...but don't we all?