The Battery Thermokruzhkus Mug is a classic black ceramic mug with a white outline of a battery indicator on it. Pour in your hot liquid – anything over 96.8F (36C) – and watch the green cells within the battery light up. As your beverage cools the battery will fade into emptiness, reminding you that you need a refill. The mug is available at ThinkGeek for $14.99.