Real-World Computing
Random numbers
Believe it or not, generating truly random numbers is a difficult job for a computer. The problem is that computers are deterministic - they only do exactly what they're told. Computers can generate pseudo-random numbers very efficiently, but for some applications such as cryptography, less predictable random numbers are required.
Every method of generating true random numbers on a computer involves some kind of external input. This can be as simple as keyboard typing speed or mouse movements, or non-interactive input such as network activity. More esoteric sources of input include radioactive decay, radio antenna, and digital image capture.