Fred and Wilma are laying in bed with Pebbles and Pebbles looks down and sees Fred's penis and says "What's that daddy" and Fred says "That's my rock".
Pebbles thinks for a little while and then looks down again and sees Wilma's vagina and says "Mommy whats that?".
Wilma says "Thats my rock grinder"
Pebbles thinks for a while and then says "Oooo I get it, so daddy puts his rock in momma's rock grinder and out comes Pebbles"