What's a "bathing suit?"

I was in the sportswear department the other day, waiting in line at the cash register. I heard a little voice ask,

"Mommy, is that a bra?"
"No, it's a bathing suit."

"Mommy, is it underwear?"
Again, "NO. It's a bathing suit!"

"Mommy, it's not underwear?"
"NO!"

Out of the mouths of babes.