From Thinkgeek:

Caffeine. It makes us light up. It excites us. The thought of that first cup of coffee can really get us moving in the morning, literally and figuratively.

Much as caffeine particles pass into our bloodstream and make us bounce off walls, so, too, 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 these particles collide with oxygen in particular at lower altitudes (up to 150 miles), the photon released appears green or yellow, giving a similar light show as to the one captured on this mug when you fill it with warm liquid.

[Aurora Borealis Heat Change Mug]