This has to be one of my all time favourite
A lad walks in to his parents, bedroom to find his dad giving his mum one. The dad laughs throws a pillow at the lad and tells him to get out.
Hours later the dad hears a commotion coming from the lad's bedroom. He enters the bedroom to find the lad giving his gran one. The dad looks horrified. "Not so funny when it's your fucking mum is it...?" says the lad... PMSL gets me very time ;)
What's your favourite?