 The view that natural kind terms are rigid designators has consequences for identity statements involving natural kind terms (e.g., “brontosaurus is apatosaurus”). The reason is that any identity statement relating two rigid designators is necessarily true. Hence identity statements involving natural kind terms express necessary truths, even if knowable only a posteriori. Kripke regards theoretical identities such as ‘water = H2O’, where one of the terms is a description, as instances of such a posteriori necessary identities. Such a claim requires an argument to show that ‘H2O’ is a rigid designator, since it is not a directly referring expression. We may take ‘H’ and ‘O’ to designate hydrogen and oxygen rigidly. However, if the formula ‘H2O’ just means ‘hydrogen and oxygen compounded in the ratio 2:1’ one might wonder whether, if the laws of nature are contingent, there might be worlds where hydrogen and oxygen are compounded in that ratio in such a way that the resulting compound does not belong to the same kind as H2O, and is not water. If that is possible, then ‘H2O’ is not a rigid designator, and ‘water = H2O’ is not necessary. (As noted above, Putnam’s Twin Earth thought experiment does not establish the identity of water and H2O—all it establishes is that being constituted of H2O is a necessary condition of being water. In which case it is consistent with some instances (actual and possible) of H2O not being water.) Most discussions accept that to count H2O as a substance it must be compounded of hydrogen and oxygen in the way that they are actually compounded. The rigid designation of simple, directly referring names and kind terms is often explained in terms of their having an indexical character. In ideal cases where the name is introduced via an ostension of its intended referent, the designation is literal. In other cases indexicality is less apparent, as we see in the case of ‘water’ where there is something like an implicit ostension of ‘watery stuff around here’ (though ‘water’ could not mean ‘watery stuff around here’, since then the referent of ‘water’ would then be different in an XYZ environment). A similar implicit indexicality is found in the theoretical specification of kind terms, which explains their rigid character also.