Before SI, every region had its own units, and converting between them meant memorizing a new number for every pair. In the 1790s, French scientists built one system on the number our counting system already uses: ten. They defined the metre, then made every other length by scaling it by tens: 1 kilometre = 1000 metres, 1 metre = 100 centimetres, 1 centimetre = 10 millimetres. The same ladder works for grams and litres, unchanged. Say it plainly: in SI there is only ever one question - 'how many tens?' - and the prefix answers it.

A pharmacist measures a dose in milligrams, a truck's load in kilograms, and a patient's height in centimetres. What one fact about SI makes all three measurements easy to compare? -> All three are powers of ten of a base unit, so the same x10 ladder converts any of them.