E.V.
Hi J.. You've gotten tons of great responses, but I have to add mine. We have a 4 month old and my son will be 4 next week. When I was about 8 months pregnant and not sleeping, I was lying in bed with the TV on watching what you shouldn't watch, "maternity ward" or something like that. Well, I had fallen asleep (it was late) but when I woke up, I found him standing next to the bed with his mouth open staring at the TV. By this time, we had already named the baby, so he asked "mama, are you going to poop Lilly out?" I didn't know what to say, but I finally just said "well, no, mommy's have another hole that is made for babies to come out of, but it is similar to that". And that was all it took. Now, I was laughing about this and telling the story at work and one of my students (a college sophmore) looked very confused...he asked point blank - "wait a minute, which hole does it come out of then?" and he was very serious.
So, after that, I decided that my kids will have a very serious talk as soon as they can understand so they aren't 19 and wondering which hole is which!
Good luck to you~