This little thought just crossed my mind. During day to day Windows usage, my CPU fan stays relatively quiet, yet it is supposedly still working at 2GHz. Then if I run a program that performs some complex calculations and doesn't voluntarily yield its timeslice, the CPU fan starts to sound like a vacuum cleaner.
But what is actually happening that's creating excess heat? The CPU is still running at 2GHz, yet the fact that it's not in an OS's idle loop (which still has to be executed AFAIK) means the temperature shoots up. Maybe it's elementary electronics or something, but I don't get it.