I'm crap at this sort of thing so I asked the internets and this one surprised me -
Four jolly men sat down to play,
And played all night till break of day.
They played for cash and not for fun,
With a separate score for every one.
When it came time to square accounts,
they all had made quite fair amounts.
Now, not one has lost and all have gained -
Tell me now, this can you explain?