To further add to this, a unit would be something basic like litre, metre, or a gram. So 1000 litres is a kilolitre. 1000 metres is a kilometre. 1000 grams is a kilogram. You may be familiar with the computer byte. A kilobyte is 1000 bytes. A megabyte is 1000 of those. Everything is divisible by 10, and everything makes sense.

Interestingly, even though a calorie isn’t a metric unit (the joule is), the energy to raise 1 millilitre of water by 1 degree Celsius is 1 calorie.

Also, 1 gram of water is 1 millilitre. And if you measure that in size, that’s 1 cubic centimetre. So if you go buy a litre of water, you know it’ll be 1000 cubic centimetres, and it’ll weight 1kg.