Double Boilers
A double-boiler is used for heating a pot to melt the contents, but without putting the pot in direct association with the heat source.
The problem is such: When melting cheese (fondue) or caramel (cheesecake), etc, you need to melt a reasonably large portion of whichever, which means it stays on the heat for a while. Thus whatever is on the bottom of the pan will get hotter and burn on. Even with constant stirring, the bottom of the pan will scorch and stick. And the scorched flavor gets into the food, and is hard to clear off the pan after.
The solution: A double boiler uses two pans. One is filled with water (or similiar), which is boiled. Duh. The second pan is set inside the first, and your materials are placed in this pan. The boiling water (and steam) transfers the heat to the inner pan, but without a heat-source, so the scorching is “eliminated” (or vastly reduced at any rate). Yay! The pans are designed so that they actually fit together, enclosing the water and steam.
Cheap hack: If you have two sauce pans that are close in size, you can set one in the other for this. As you add materials, and especially when you’re stirring, the upper pan will sink and you’ll slosh out water. And then there’s the part where you’re sticking your arm into steam. But it works if you don’t have a real double-boiler.