Okay kiddo, let me explain. Boys and girls have different body parts used for making babies. For boys, their sex organ is called a penis. It is what they pee out of, but it's also used during sex. When a boy gets excited, his penis gets hard and can go into a girl's body part called a vagina. That's how babies are made!
For girls, their sex organ is called a vagina. It's a little bit like a tube inside their body that connects to their uterus where a baby can grow. Girls also have something called a clitoris, which is a little button that feels good when touched. During sex, the penis can go into the vagina and sperm from the boy can swim up to fertilize an egg from the girl, which can lead to a baby growing in the uterus.
It's important to remember that sex is something private that should only happen between two consenting adults who love each other and are ready for the responsibility of having a baby if that happens.