Two identical twins call different liquids 'water' — and neither one can tell

Hilary Putnam imagined a planet identical to Earth in every way, except its lakes are filled with a liquid, XYZ, that looks, tastes and behaves exactly like water but has a different chemical formula. Before chemistry existed, an Earthling and their molecule-for-molecule identical Twin Earth counterpart would have had identical thoughts on saying 'water' — yet the word would refer to two different substances. Putnam used this to argue that meaning isn't purely in your head.

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