A Red headed woman walks into her teenage daughter’s room and finds cigarrette butts. And she says “Wow I didn’t know my daughter smoked”

A Brunette woman walks into her teenage daughter’s room and finds an empty beer bottle. And she says, “Wow, I didn’t know my daughter drank”

A Blonde woman walks into her teenage daughter’s room and finds a used condom. And she says, “Wow, I didn’t know my daughter had a penis”

My informant first heard this joke in high school from one of her friends. She went to high school in California’s Central Valley, which is a fairly conservative area in the state. But her high school was a very small one filled with students that needed more of an academic challenge in school than the local public high schools provided. The joke is funny because of it’s twist at the end. The first two mothers drew logical conclusions from what they found in their daughters’ rooms. We expect the third mother to do the same and say “Wow, I didn’t know my daughter has been having sex”, but instead she draws an absurd conclusion that she should already know to be false. Which is “Wow, I didn’t know my daughter had a penis”.

I agree with my informant’s explanation as to why the joke is funny. But aside from the obvious surprising twist at the end it also draws on the blonde women are dumb stereotype. It is the blonde woman who sees a condom and assumes that her daughter must have a penis. Both the red hed and the brunette make logical assumptions.