Challenge coin rules differ among military units, but the basic rules remain the same. One person initiates the challenge with a coin check by slapping a table and laying down a coin. The other members of the unit then show their own coin. If a member cannot produce a coin, that person buys drinks for the rest of the unit and if everyone shows their coin, the challenger buys the drinks.
The challenge coin serves to recognize unit members and boost morale. It is a sign of recognition for a special achievement by the unit. Each challenge coin is stamped with the unit's insignia or emblem and is supposed to be carried at all times.
One of the most famous challenge coins is the Bull Dog. It was given to the U.S. Air Force's B-52 tail gunners when they graduated from their technical training. The coin depicts a Bulldog and signifies strength and courage.