A guy walks into a bar, desperate for the toilet. He sees the sign, "Toilets upstairs" so he dashes up, checks every room, but for some reason, he cannot find the toilet. All he can see is a hole in the floor, and at this point, it's seconds away from brown trousers time, so he thinks "better to do this than lay one out on the carpet", so he squats over the hole and lets it fly.

Feeling kinda guilty that he just dropped a log in the landlord's floor, he decides to buy a pint, but as he comes down the stairs, he realises that the packed pub is now completely empty except for the barman slowly rubbing a glass.

He asks "What happened? Where did everyone go?"

The barman gives him a stern stare and says "Where were you when the s**t hit the fan???"

(If you don't get this joke, then you need to read up on this terminology).