The one about making babies...

Dad came home one day in an exceptionally horny mood and took his wife upstairs for sex. Just when they were really getting into it, their young son entered the room and started to cry.

"What's wrong, son?" the father asked. "Why are you crying?"

"You're hurting my mummy," the little boy replied.

"No, no," the father reassured, "I'm not hurting her. We are making babies."

This seemed to calm the boy, and when he left the room the couple went back to their business.

The next day the father came home from work and found his son on the steps, crying.

"What's the matter NOW?" asked Dad.

His son replied, "It's those babies you were making with mummy yesterday. The postman is upstairs eating them!"