What Does Gluten-Free Even Mean, Anyway?

I have a friend who is gluten-free, and I have no idea what that even means for her, other than she cannot eat my delicious slice of pizza because there's wheat in it (or something).

Well, Jimmy Kimmel busted several gluten-free would-be types on his show the other day, asking them what in the world gluten is. Hilarity ensues.