Full Idea
The difference in the two types of 'water' in the Twin Earth experiment seem psychologically irrelevant, for behaviour causation or explanation.
Gist of Idea
Two types of water are irrelevant to accounts of behaviour
Source
Jaegwon Kim (Philosophy of Mind [1996], p.203)
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.203
A Reaction
A rather important point. No matter how externalist you are about what content really is, people can only act on the internal aspects of it.