Once the atoms are trapped in the magnetic field, the gas has an average temperature of about a millikelvin. But, the speed of individual atoms varies. When Zwierlein applies a certain radio frequency to the trapped atoms, it essentially acts as a razor—shaving off the hotter atoms and leaving the cooler ones. The remaining atoms will then reach a lower average temperature and Zwierlein can repeat the process, reaching lower and lower temperatures. This process is called evaporative cooling.