Challenge Coins

About the challenge: "Challenge coin rules only apply to other individuals who also have a challenge coin. A holder of a challenge coin may "challenge" any individual who is known to have a coin. A challenge is made by withdrawing a coin and raising it in the air or by tapping it on a bar or table. The individual who is challenged is required to produce their coin within 60 seconds. If the individual produces the coin, the challenger is obligated to buy them a drink. If the challenged individual fails to produce the coin, they are obligated to buy the drink. The reward does NOT have to be an alcoholic beverage. It can be a soda or any other reward that the two individuals agree on. If a coin is dropped and it hits the floor, the ownder is obligated to buy drinks for anyone who hears or sees the coin hit the floor (provided they have their coin on them). Coin challengers are known to strike anywhere at anytime. They insidiously stalk the challenge, waiting for just the right moment to attack. An innocent bystander may never hear the challenge - only the challengee's despairing cry, "... Ah ____! I forgot mine!"
Taken from: globalsecurity.org
It also tells two stories about the history of these coins.
The following coins were presented to individuals serving in Berlin.

Image of Berlin Brigade Challenge Coin. Berlin Brigade Image of Berlin Brigade Challenge Coin.
Image of Berlin Brigade Challenge Coin. Berlin Brigade Image of Berlin Brigade Challenge Coin.
Image of Military Police Challenge Coin. Military Police Image of Military Police Challenge Coin.
Image of Field Station Berlin Challenge Coin. Field Station Berlin Image of Field Station Berlin Challenge Coin.