A bloke on his own; in a strange town fancies a drink. So he finds a pub he likes the look of and goes in. He orders a pint an stands at the bar drinking. Half way down his second pint he needs the loo. He looks around. There are a few others in the bar but are in groups chatting, playing pool or on the fruit machine.
He takes a beer mat and writes a message on the mat. "Do not drink this beer; I spat in it".
He goes to the loo and returns 5 minutes later.
He goes to take a sip of his beer when he sees the beer mat and his message.
Added to his note is. "So have i".