That is, the trick is dirty (dishonest); it probably works in clean bars as well as dirty bars:

Bet someone they can’t balance a full beer with only their thumbs. The trick is to have them only place their thumbs on the edge of the bar. Now you take a full beer and balance it on their two thumbs. When they succeed, say “Well, guess you can” and walk away. They will spend a few minutes or more trying to get the beer off their thumbs.