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!"