Period.

I was talking with my hygienist today...well, she was talking, I was lying there with my mouth open and my teeth being picked out of my head. 

Anyway. She was telling me that her kids were doing homework at the table together. Her daughter announced that all the fourth grade girls were going to a special class to learn about their periods. 

Her son replied, 'That's silly. Why have a special class only for girls? Everyone knows a period comes at the end of a sentence.' 

I'm still chuckling.