The Butt of the Joke

Laura's Post:

S: (reading her book aloud) That's the word "but." (giggles)
Me: (trying to take the high road) Yes. The other "but," the one you're thinking about, is spelled differently with two "T"s. B - U - T - T.
S: (giggling out of control) TOOTIES! Ha ha ha ha!...

(OK. I give up. It's pretty funny. I wish I still got such joy from bodily functions.)