From Thinkgeek:

The two mugs in this set look identical, but when you add hot liquids, one of the mugs slowly reveals a live cat, and the other, dead. But until you add the hot liquid, you don’t know which mug you have, and the cats can be simultaneously alive and dead, just like in Schrodinger’s thought experiment. No radioactive source or poison necessary.

[Schrodinger’s Cat Heat Change Mugs: Set of 2 (Dead or Alive)]