Two little boys go into the grocery store. One is nine, one is four. The nine-year-old grabs a box of tampons from the shelf and carries it to the register for check-out. The cashier asks "Oh, these must be for your mom, huh?"

The nine-year-old replies, "Nope, not for my mom."  Without thinking, the cashier responded, "Well, they must be for your sister then?" The nine-year-old responded, "Nope, not for my sister either."

The cashier had now become curious. "Oh. Not for your mom and not for your sister -- then who are they for?"

The nine-year old says "They're for my four-year-old little brother."  The cashier is surprised: "Your four year-old-brother?" 

The nine-year-old explains: "Well yeah, they say on TV if you wear one of these, you can swim or ride a bike -- and my little brother can't do either of those things."