Heading come true

"She's the bee's knees!"

Do bees really have knees? Where does this expression come from? I know what it means, but who the hell thought up this expression? So many questions, so few answers.

I know I could probably query the Google god, but I just wanted to throw that out there and not think about it again. Why not, "She's the tiger's teeth!" or "She's the llama's lips!". After all, bees are not very friendly, loveable, or sexy.