Someone in the family was pregnant, and my nieces were putting together the facts related to baby making at the time. A light bulb went off and they noted 'oh, so that means so and so had s*x.' Then, another light bulb went off and they said to their mom "oh, so that means that you and dad have had s*x too!"
They seemed skeptical, and when further questioned, said "Dad just doesn't seem like the type of person that would have s*x."