My son caught me off guard with some of this when he was 3. While we were exiting the zoo, he noticed that there were a lot of couples there who didn't have kids. At this point, he still thought of couples in terms of the "mom" and the "dad" and didn't know what to make of a "mom and dad" with no kids. He told me he thought all "moms" had kids. When I told him no, not all people want to have children, he asked something along the lines of "don't their bodies make babies anyway?" Not knowing what else to say, I told him that if women don't want to have kids they can take a pill and it stops their body from making them. Within seconds, he got a huge smile on his face, stopped in his tracks and said (a little too loudly for comfort), "Now I know why you take a pill every day! You don't want to have any more kids!" Looking over my shoulder for anyone who might have been within earshot, I quickly ushered him to the car, feeling like I'd been outsmarted by a three year old.
