On a planet just like Earth, the stuff everyone calls 'water' isn't H2O

Hilary Putnam imagined a Twin Earth, indistinguishable from ours, except the colourless liquid everyone there calls 'water' is a different chemical compound, XYZ, that just happens to look and behave the same. Before chemistry existed, people on both planets used 'water' identically and believed identical things, yet Putnam argued their word already referred to different substances, because meaning depends on the actual stuff in the world, not just on what's going on in a speaker's head.

— Hilary Putnam, The Meaning of 'Meaning' — Minnesota Studies in the Philosophy of Science, 1975