 Putnam argues that the stuff that falls from the skies and fills the lakes on Twin Earth is not water. According to Putnam, when people used the term ‘water’, even in 1750, they intended to refer to a natural kind, a kind of thing whose instances share a common nature, not directly observable, which explains the observable properties of instances of the kind. They identified water by observable characteristics like colorlessness and odorlessness, but they also assumed that there was a microstructure which explains these observable properties. Since 1750, we have learned what this microstructure is, namely that water consists of molecules of H2O. But water was H2O even in 1750, before we learned this. (Other natural kind terms work in the same way. For instance, we identify diseases by their symptoms, but we assume that there is an underlying cause of these symptoms, for example a particular microorganism, and that even before we know what this underlying cause is, it makes the disease what it is.)