The most official water costs $120k a gallon

We all learned in science class that water freezes at 0 °C or 32 °F at atmospheric pressure. But what water, exactly? Even after you distill out all the dissolved salts and minerals to get pure H 2 O, not all H 2 O is created equal – and for the most precise temperature measurements, the differences really matter.

Here’s the problem: hydrogen and oxygen atoms aren’t all identical. All atoms of a single element have the same number of protons by definition, but they can have different numbers of neutrons, forming different isotopes. We need to know how much of each isotope to use for our experiments.