Full Idea
The word 'water' used on Twin Earth refers not to water but to this other liquid (XYZ). Yet there is no relevant difference in the mental state of Twin Earth speakers and speakers on Earth (in 1750) to account for this difference of reference.
Gist of Idea
'Water' on Twin Earth doesn't refer to water, but no mental difference can account for this
Source
Hilary Putnam (Reason, Truth and History [1981], Ch.2)
Book Reference
Putnam,Hilary: 'Reason, Truth and History' [CUP 1998], p.23
A Reaction
In this world, if you and I separately meet twins, and I think about this twin while you think about that one, our mental states are different even if they are indistinguishable. I know I'm thinking about my twin, not yours. Indexicals.