We imagine there is a planet, ‘Twin Earth’, exactly like Earth in all readily observable respects. Every person and thing on Earth has a twin equivalent on Twin Earth. However, there is no H2O on Twin Earth. Instead, there is a liquid that shares the manifest properties of water (e.g., its being colourless, tasteless, odourless, good for drinking, etc.). This liquid is composed not of H2O molecules but of very different molecules to which we assign the made-up formula ‘XYZ’. Despite its superficially water-like properties, intuitively XYZ is not water. What prevents XYZ form being water is the fact that XYZ is microstructurally different from H2O. If intuition is reliable in this respect, then (i) possessing water’s superficial properties is not sufficient to make something a sample of water, and (ii) being composed of molecules of H2O is necessary for something's being a sample of water. (via stanford encyclopedia of philosophy)what putnam wants to say is that H20 is a natural kind.