My apologies if this is an old joke. I just found it this morning.

One day the presidents of Budweiser, Miller, and Guinness go out to dinner together. The president of Budweiser orders a Bud, the president of Miller orders a Miller and the president of Guinness orders a Coke. The waiter expresses surprise and asks why. The president of Guiness says, "Well if these gentlemen aren't having a beer, I don't see why I should either".