Water is too hard to keep constant, and too hard to reproduce. Keeping it exactly in 1dm3 is tricky, for example. Not for "regular" use, with a few significant

Water is too hard to keep constant, and too hard to reproduce. Keeping it exactly in 1dm3 is tricky, for example. Not for "regular" use, with a few significant digits of course; but when you're defining fundamental physical constants, atoms count.