I found this on the Naruto forum. Basically you write what characters would say in the bathroom. Ever been in a public bathroom? Pretty gross, but funny to read. I guess you should have one saying per post?
Examples:
Katara walks into the bathroom to read "For a good time, call Sokka with a belch and a whistle." She walks out to beat him up.
--OR--
Zuko walks into the bathroom and sees a crude drawing of him and the words "Zuzu is a weak pansy. Love, Azula" underneath it. He then procedes to make plans for revenge.
Have fun?