Think Geek explains that it is the caffeine that makes the mug light up. The effect is exciting! In fact, that very first cup of coffee can get us moving and make us truly happy.
The scientists explain the process like that: “Much as caffeine particles pass into our bloodstream, so the particles from solar winds pass through the Earth’s magnetosphere near the poles and share energy, causing a spectacular display in the upper atmosphere. When a collision happen between the particles and the oxygen at particular lower altitudes, the photon released appears yellow or green, making a similar light show as to the one captured on the mug when filled with warm liquid.”