Little Johnny walked into his parents bedroom and caught his Dad giving one to his Mum. His Dad just laughed out loud, threw a pillow at Little Johnny and told him to get out.
About half an hour later, Dad could hear a hell of a commotion coming from Little Johnny's room. He went to see what was going on, and saw Little Johnny having sex with his Grandmother.
Little Johnny looked at his Dad and said, "not so fucking funny when it's your Mum, is it?"