the fickle liter (or "litre" in the rest of the English-speaking world)

We take the constancy of some things for granted. The speed of light, the love of a dog, the metric system. But!

Of course, a liter is a cubic decimeter (dm³); that's why a cc (cubic centimeter) equals an ml (milliliter).

But did you know that, from 1901 to 1964, a liter was not one, but rather 1.000028 dm³? 

You could, as they say, look it up.

[Turns out that the liter is like the hour and the day. It it not really an SI unit; it is a non-SI unit that the SI will accept.]