 The most famous experiment imagines a remote planet called “Twin Earth,” a planet which is superficially indiscernible from Earth (so that even its events parallel the events on Earth). But there is one key difference: The compound H2O occurs nowhere on Twin Earth. In its place is a different compound that looks, tastes, etc., exactly the same, which we may call “XYZ”. Now consider Oscar, a normal adult English speaker, along with his Twin Earth doppelgänger, Twin Oscar. (Ignore that people are mostly H2O; that’s an accident of this example.) The externalist intuition is that, even if these “twins” are unschooled in chemistry, Oscar uses ‘water’ to refer to H2O yet Twin Oscar uses ‘water’ to refer to XYZ. After all, when Oscar asserts “water is scrumptious,” this is directed at water, yet Twin Oscar’s parallel assertion is in reference to XYZ, a.k.a. twin  water.[6]