Little Johnny walks into his parents bedroom one night and sees his dad giving it to his mum. His dad just laughs, throws a pillow at him and shouts "Get out!"
A little while later, Johnny's dad hears a commotion coming from Johnny's bedroom. He rushes in and is horrified to see Johnny shagging his granny.
Little Johnny just looks at him and says, "It's not so fucking funny when it's YOUR mum, is it?"