[oblomov]

I thought that 1 l of pure water was supposed to have mass of 1 kg (to be precise, weight is measured in newtons, not kilograms)? And that 1 l of water has volume of 1000 cm^3?

[Jedi Master Pikachu]

1mL, 1g ? (which does convert into 1L, 1kg).

[Tanniker Smith]

I guess that that wasn't immutable enough. Maybe because the water has to be at precisely 4 degrees Celsius besides (which is whatever Kelvin).

A precise amount of a pure substance at a particular temp is a little more complicated then the time light travels a certain distance (a sec) or the length of several billion cycles of some isotope.

Me? I'll stick with miles per hour.
TS
Yo! Where's my metric hour? Why am I still using the stuff from Ancient Babylonia???