Water, or water?
Water is made up of two hydrogen atoms and one oxygen atom. These atoms can be configures in two different way to make different isomers.
As they are nearly identical, they have no effect on us whether we drink one or the other (water is usually composed of both types of isomers).
However, these two different waters can behave differently in delicate research experiments. Having the two separated could make for more accurate testing and results.