system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did we decide the value of a single unit, such as 1 watt or 1cm?
gpt
'We' didn't, someone did once and it got popular. Often the popular unit was at least a bit reworked later, like the second, but still much the same.

Most units are defined in terms of other units. A Newton, for example, is 1 kilogram times 1 metres per second squared.

The 'base' units tend to be fairly arbitrary. Someone just decided something. The kilogram, for example, is based on the 'prototype' kilogram object. That prototype is, by definition, 1kg in mass.

As long as people reasonably agree on it being the base unit it's all good.