system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do CPUs and GPUs need fans and intricate cooling apparatuses, and where is the heat coming from?
gpt
Electrical resistance. They get hot for the same reason the heating elements in an oven do.

However, there is an additional factor involved. Calculation is, on a fundamental level, a form of work. You're flipping physical bits from one to zero and back, and that has a cost. In this case, that cost is expressed in the form of waste heat. Entropy will always have its price.

You need to cool them down both because they could get so hot the circuit boards melt, and because increasing the local temperature increases the minimum amount of energy necessary to flip a bit. The computer will get slower at higher temperatures, so keeping it cool is also keeping it functional.