Spit on my beer

One evening, Frank was drinking at a bar when the bartender came over to tell
Frank that he had a telephone call.

Frank had just bought another beer and he didn't want anyone else to drink it.
So, Fred wrote a little sign and left it by his beer that read: I spit in my
beer.

When Fred returned to his bar stool, there was another note beside his
beer: I spit in your beer, too!