The teach your kids proper names for body parts post reminded me of what a FB (and former classmate) friend posted a few nights ago.
She's asking for advice for her 4 year old son, who asked her how she pees without a penis. She's proud of him because "he's highly inquisitive for his age". Her post goes on to say "I'm a doctor so I should be able to answer this, but would like advice from other parent friends." Advice ranged from getting an anatomy book, to drawing figures, to just saying that that's how you're made and leave it at that. But the best, and the one that she settled on as the best and what she's going to tell her kid is "First, does he know about the urethra?" Yes. "Good, then explain that the urethra is your penis, only it goes inside of you and that's how you pee. From your internal penis. He has an outside penis, you have an inside penis."