I remember when I was like 10 or 11 my bestfriend's brother who was like 14 at the time sat us down and told us what sperm and semen and all that stuff was. They had this big stuffed animal (I think it was a bear?) and he stripped down to his boxers and started humping it and told us that was how you have sex.
And then like a year or two later with those same guys we looked up pictures of vaginas and stuff on the internet