 Putnam’s most famous examples involve “Twin Earth,” an imaginary planet which is molecule-for-molecule identical to Earth, including having exact duplicates of Earth’s inhabitants, except for a systematic change in certain parts of the natural environment. In a particularly well-known version of this example, we consider Earth as it was around 1750, before the chemical structure of water was discovered, and we consider an inhabitant of Earth named “Oscar” who is a competent user of the term ‘water’. We then imagine a Twin Earth which is exactly like Earth in every way, including having an exact duplicate of Oscar, with one exception: for every place on Earth that contains H2O, the Twin Earthly duplicate of that place instead contains XYZ, a substance with a different microstructure from water but with similar observable properties. On Twin Earth, it is XYZ, not H2O, that falls from the skies and fills the lakes and oceans.