I overheard this recently when I was out with one of my friends at a Bar-B-Que. The people's house we were at that were hosting the party has the most adorable girl, and this is what she said...
The 7-year-old girl went to her dad, who was cooking in the back yard. She asked him, "Daddy, what is sex?"
The father was surprised that she would ask such a question, but decides that if she is old enough to ask the question, then she is old enough to get a straight answer.
He proceeded to tell her all about the 'birds and the bees'. When he finished explaining, the little girl was looking at him with her mouth hanging open. The father asked her, "Why did you ask this question?"
The little girl replied, "Mom told me to tell you that the stuff would be ready in just a couple of secs."