Little Johnny walks into his parents bedroom to find his Dad giving his Mum one. His Dad smirks and throws a pillow at the door saying, "Get outta here, you little shit!"
A couple of hours later Dad hears a whole lot of commotion coming from little Johnny's bedroom. He goes up to find little Johnny giving his Grandma a right royal seeing to.
Little Johnny smiles, "It's not so fucking funny when it's YOUR mum, is it?"
Courtesy of Sickipedia.org: http://www.sickipedia.org/sex-and-shit/incest#ixzz3w6g7iTsy