A childs revenge.....

A little 8 year old lad hears some odd noises one night and so walks into his parents bedroom to investigate.

On arriving he sees his father going hell for leather into his wife from behind.
The father looks round & smiles at the lad and carries on merrily humping away.

A few minutes later there's an awful blood-curdling cry from grandmas room - the father runs like hell into the old ladies room and sees his son banging away in the octagenerians arsehole.

Little lad looks up and calmly says "aint so ****ing funny when its your own mum, is it?"