Mommy what are these????? Ahhh man

Nathan and Katelyn decide it's still summer and lay some towels down so they can soak up the sun. As Nathan lays there, shirtless, Katelyn asks, "mommy what are these?" I look at what she is pointing at, Nathan is also looking at me waiting eager to hear what I am going to say.  I reply " his chest" , " "no mom these".  Katelyn finds Nathan 's nipples interesting.  She says, "are they boobies?"  Ahh man. I don't want to introduce the word nipple just yet. I don't want the word repeated somewhere else, and  I get those looks.  I am trying my best to come up with something they can both understand.  Katelyn, that is Nathan 's chest. Boys chest don't grow. Only girls boobies girl. Ok can we talk about something else other than body parts.