What does a bra do?

Sam came walking out of our bathroom area and had my bra around his neck. He says,

"Hey mom, what is this?

I looked at him and tried not to laugh and answered him, "It is a bra."

He says, "what is a bra?"

I say, thinking to myself, good question, how do I answer this.... "Well girls wear them on their chest."

Sam, "Why, what does a bra do?"

Stumped really, I say, "They keep things where they need to be."

Sam, "Oh."

I seriously should have gave him a better answer I suppose, I just thought it was pretty funny. He has seen them before, he will wrap up his stuffed animals with them and all sorts of things, but this was the first time he decided to find out their true use!