Challenge Coins
The coin in a service member's pocket is a badge of belonging — and pulling it out (or failing to) at the bar can cost you a round.
How it actually works
A challenge coin marks membership in a unit or personal recognition from a leader — commanders, senior enlisted, and even Presidents hand them out as a wordless "you did something that mattered." Then there's the coin check: someone slaps their coin on the bar, and everyone present has to produce theirs. Whoever can't buys the round; if everyone can, the challenger buys. U.S. troops stationed in Germany after WWII borrowed the mechanic from the local "pfennig check." There are no formal, service-wide rules — every unit runs its own version — but the coin lives in your pocket for a reason.
The beloved origin story — a WWI pilot shot down and captured, who escaped and was saved from execution by a French unit only because his squadron coin proved he was an American airman — is almost certainly legend. It's repeated everywhere and documented almost nowhere; the tradition more reliably traces to the mid-20th century. The coins are real, the culture is real, the founding tale is folklore.
Popularly dated to WWI (unverified); tradition solidified mid-20th century, spread to all branches and beyond