Coins

Dev Barros had worked the night shift on Callisto for three years, which meant that for eight hours every night he was the only person awake on the whole moon. He watched the air gauges and the power gauges and the seismic sensors, and at 0300 he walked to the break room and fed coins into a vending machine that had been shipped out from Earth in 2091 and never forgiven anyone for it.

The machine ate his coins about one night in four. He had learned to kick it in a particular spot, low on the left, which worked about half the time. Nobody else on the base knew about the spot. It was the closest thing he had to a secret.