Little Johnny walks in on his parents one night whilst they are (ahem) 'Doing the Deed'.
"Get out, son" smirked his dad, throwing a pillow at him.
A little later on, dad hears a commotion from Little Johnny's room. He goes in and is shocked beyond belief to find him going at it hammer and tongs with his Gran.
Little Johnny looks up at his father, grins and says "Ha! It's not so funny when it's your f****ng Mom is it??"
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"If I'm not grounded pretty soon....I'm gonna go into orbit" Warren Oates AKA 'GTO', from the movie 'Two-Lane Blacktop', 1971