Potty jokes can be annoying, but at least they're harmless. Once kids get close to their middle-school years, they use humor to discover who they are, says Joel Goodman, director of The Humor Project in Saratoga Springs, NY.

Tween boys usually lean toward slapstick (think Jim Carrey or Ben Stiller), anything gross (farting, burping, boogers), and the inappropriate (sexual innuendo, violence, and rude talk). "Most boys love the dumb stuff and testing boundaries," notes Goodman.

Tween girls tend to joke about relationships, either between friends or the romantic kind, and they'll experiment with various put-downs in order to see where they fit in at school (is she a mean girl, a tomboy, or a geek?).

Your response: Point out when your kid crosses the line. Explain that a joke shouldn't embarrass or make others uncomfortable, and that he can't cover up an insult simply by tossing out "JK!" (just kidding).