how does the metric system work?
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There are a few key principles of the metric system.
It starts with “base units.” It takes what are thought of as the core physics concepts – time, length, mass, electric current, thermodynamic temperature, amount of substance, luminous density – and assigns each of them a unit ([these days each defined in relation to one or more core constants](https://en.wikipedia.org/wiki/File:Unit_relations_in_the_new_SI.svg) and each other).
All other units are derived from and defined in terms of those base units, and must be ‘coherent’ – meaning that there are no scale factors involved. For example, once we have the metre for length and the second for time, we get the unit for acceleration as the metre-per-second-squared. Once we have acceleration and mass, we have the definition of force as mass multiplied by acceleration, so our unit for force (the Newton) must be the unit for mass multiplied by the unit for acceleration (1N = 1 kg * 1m/s^(2)). Now we have the Newton for force, we have energy defined as a force multiplied by distance, so our unit for energy (the Joule) must be 1 Newton * 1 metre. And so on.
The other useful principle is that we have a system of pre-fixes for getting different orders of magnitude. If you look at, for example, the [traditional English units for length](https://upload.wikimedia.org/wikipedia/commons/9/9e/English_Length_Units_Graph.svg), even with just the simple ones you have 12 inches in a foot, 3 feet in a yard, 1760 yards in a mile. You have 2 yards in a fathom, and 8 furlongs in a mile. Each of these has a different conversion factor, and a seemingly random name. The metric system works on powers of ten; you have the metre, the kilometre (1000m), the centimetre (1/100m), the millimetre (1/1000m) and so on. Given any unit you can get another by multiplying by or dividing by 1000, and changing the prefix to the next one.
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