Challenge coins have a rich tradition of camaraderie and identity, carried and exchanged by astronauts and soldiers. When Long Now built The Interval, we made our own challenge coins, which quickly became the most popular reward for our "brickstarter" fundraising campaign. Each coin reads “Carpe Millennium” on one side and features a bristlecone pine on the other. The challenge is initiated when one holder slams their coin down on the bar — anyone who can't produce their own coin is on the hook for a round of drinks.