Here's one from when I was about 9 or 10. Younger DiSers may not remember the Schweppes adverts that inspired this hilarious joke.

A man ejaculates into a bottle of Schweppes tonic water, screws the lid back on and leaves it on a wall.
Later that day a woman is thirsty and spies the bottle. She drinks it.
A few years later her child asks her "Mum, who is my father?"
The woman replies "Schhh, you know who."

Cue much laughter and knowing winks.