When my son was about five he evinced some curiosity about the origin of babies. So we’re explaining without trying to get too deep into the exact specifics, and I said “You know how we put seeds in the ground to grow stuff? The daddy has some seeds inside of him, and sometimes when they’re in bed, he puts some in the mommy and she grows a baby. “. He gets silent and thoughtful and you can see the wheels spinning. Then he comes up with “ But how do the seeds get in the mommy? Do they crawl across the sheets?”