"This is my baby science book."
My son found It's Not the Stork on his bookshelf and I offered to read him a section for his bedtime story. He unerringly went directly to the section on sex. I thought I did very well, not even a nervous giggle. ;-) He's been fascinated by it ever since and we've had a lot of good conversations about bodies and babies. Today he explained to me very convincingly that a women could not have a trampoline in her uterus, no matter what it might have felt like.