They are [ame="http://en.wikipedia.org/wiki/Challenge_coin"]Challenge coin - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/File:USMCBDayCoinHeads.jpg" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/en/thumb/8/87/USMCBDayCoinHeads.jpg/180px-USMCBDayCoinHeads.jpg"@@AMEPARAM@@en/thumb/8/87/USMCBDayCoinHeads.jpg/180px-USMCBDayCoinHeads.jpg[/ame].
A direct quote from Wikipedia:
"The tradition of a challenge is the most common way to ensure that members are carrying their unit's coin. The rules of a challenge are not always formalized for a unit, and may vary between organizations. The challenge only applies to those members that have been given a coin formally by their unit. This may lead to some controversy when challenges are initiated between members of different organizations and is not recommended. The tradition of the coin challenge is meant to be a source of morale in a unit, and forcing the challenge can cause a reverse effect.
The challenge, which can be made at any time, begins with the challenger drawing his/her coin, and slapping or placing the coin on the table or bar. In noisy environs, continuously rapping the challenge coin on a surface may initiate the challenge. (Accidentally dropping a challenge coin is considered to be a deliberate challenge to all present.) Everyone being challenged must immediately produce the coin for their organization and anyone failing to do so must buy a round of drinks for the challenger and everyone else who has their challenge coin. However, should everyone challenged be able to produce their coin, the challenger must buy a round of drinks for the group.
While most holders of challenge coins usually carry them in their pockets or in some other readily accessible place on their persons, most versions of the rules permit a challenged person "a step and a reach" (particularly useful if one is challenged in the shower).
Variants of the rules include the following. If you are able to steal a challenge coin, everyone in the group must buy you a drink. During a challenge, everyone in the group must buy you a drink if you are the holder of the highest ranking coin. Some units provide strict time limits to respond to a challenge."