When you drink coffee, most of the heat is in the first two or three sips. After that, you quickly lose most of the heat. By the time you've finished the cup, only a third or less of the heat remains. "If coffee was hot throughout," says David H. Schwartz, "you'd feel more energized on the second and third cups than you do now. And if it didn't lose its heat so quickly, you'd feel the same on the third cup as you did on the first. "The first thing people think about when they want a hot drink is coffee. They assume it's the best beverage for a hot drink." Coffee drinkers try to solve these problems with a mug warmer. A mug warmer is a kind of electric heating element that you put in the microwave. It heats your mug to nearly the same temperature as coffee. As a general rule, it doesn't get hot enough. So most people reheat their coffee, and that wastes energy. And coffee always tastes better freshly brewed. The solution to this problem is to drop the coffee into the mug, heat up the mug, then pour in the coffee. That requires two heating mechanisms, so the energy is wasted twice. You can tell that pouring coffee into a mug is inefficient because coffee shops use huge, expensive metal dispensers. The designers of coffee dispensers are, of course, only trying to maximize their profit, and there's no profit in saving energy. But why not let customers do their own heating? One idea would be to have customers place their mugs inside a microwave. But that's not very practical. A microwave is heavy and expensive. And most people like their coffee hot, so they tend to use it a lot. They wouldn't want to lug around a big microwave. A more practical approach is to put mugs inside insulated boxes. They would be heated by electricity, and an electric heating device is pretty cheap. But it's still an inefficient design. It loses energy every time the mug is opened, and it takes a long time to get hot. It's better to keep the mug closed until you're about to pour in the c