When doing computations, CPUs make a lot of heat. Keeping them cool is an important task. To get a sense of how CPUs are kept cool, consider your hand. If you spread out your fingers in the presence of a fan (or move your hand through the air), you will feel colder than if you clench your fist. The reason for this is because an open hand has larger surface area than a clenched hand. A larger surface area allows more cooling to take place. On a related note, if you want to conserve heat because you are outside in the cold, you should clench your fist or huddle into a ball to minimize your surface area.

Now, back to the CPU. One way of keeping a CPU cool is to attach something called a heat sink. A heat sink usually consists of a series of long and narrow metal prongs that are like fingers. A fan then blows on the CPU and heat sink in order to carry the heat away. The reason that heat sinks are so effective is that they too have a very large surface area.