system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly does a computer randomize a number? What exactly pick the output number?
gpt
A computer never produces a truly random number, it produces something that's called a pseudorandom number. A good pseudorandom number looks random, but is actually a very very long repeating pattern (trillions and trillions long).

A simple example is:

1. Pick a three digit number (the seed) 
2. Square it
3. Take three middle digets - that's your "random" number 
4. Plug that number back into 1 to generate your next random number. 

Where the randomness actually matters (e.g encryption), your computer tracks user inputs to generate entropy, this is a measure of disorder. It then uses this entropy as the seed for a pseudorandom generator.