(update: Since this photo has been popular lately, I should point out that I didn't create this, but I have been unable to determine the source.)
This was emailed to me today, and it made me laugh out loud. You can mentally edit out "blonde" if it offends you (I'm too lazy to Photoshop it) but the joke is undeniably funny. Maybe not to someone in their first Geometry class, I guess. They're just saying, "yeah, exactly!"