One day an old farmer fell asleep in the top level in a 2 level hay shed.

When he woke up, he found his son having sex with his girlfriend on the bottom level of the hayshed. He decided¹ he wouldn't disturb them, so he laid down and rested.

After a while he heard his son say, "Father, father up above. Give me strength for one last shove."

So the father, being smart, replied, "Son, son down below. Get off and give your father a go."