The meaning of underwear

I just had this conversation with Ben...

Me:  Ben, do you have underwear on?
Ben:  Yes.
Me:  You do?
Ben: Yes.  But you can't see it.
Me:  Why not?
Ben:  Because they're called UNDERwear and they have to stay UNDER your pants.

Hmm.  Glad we've got that figured out.  Now I need to figure out how to wrestle the boy into actually wearing some underwear today.