"Two Little Boys" joke

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 quipped, "Nope, not for my sister either."

The cashier had now become curious "Oh. Not for your mom and not for your sister, 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 little 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 them!"