I've seen a couple of responses saying that it is a military tradition, but nobody has actually answered what the "challenge" is, so here it is: If you're up to the challenge, you pull out your RMO (round metallic object, because just saying the C word - not that C word - can be a challenge in itself) and slap it on the bar at the O Club. Everyone in the vicinity then has to pull out their own RMOs in response. Anyone who doesn't have one on them gets to treat those who do to an adult beverage, but if everyone is properly equipped, the challenger owes a round of drinks.
RMOs are given out as unit or campaign memorabilia and typically general officers will have their own, usually something representing their command, that are different from the unit RMOs. The generals give those out in recognition or certain events and they are often highly prized. My wife got one from USAF Chief of Staff Gen Michael Ryan when she, a civilian, played a huge role in the organization and execution of the Arnold Air Society/Silver Wings national conclave. I was pretty jealous because the highest ranking RMO I ever got was from a 3-star general.