Tampons

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 mom."
Without thinking, the cashier responded "Well, they must be for your sister then?"
The nine year old quipped, "Nope, not for my sister either."
The cashier had now become curious "Oh, Not for your sister, who are they for then?"
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 an my little brother can't do either of them.