Full Idea
'Heat' is a rigid designator, which is picked out by the contingent property of being felt in a certain way; pain, on the other hand, is picked out by an essential (indeed necessary and sufficient) property.
Gist of Idea
Pain, unlike heat, is picked out by an essential property
Source
Saul A. Kripke (Identity and Necessity [1971], p.190 n19)
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.190
A Reaction
Hm. I could pick out your pain by your contingent whimpering behaviour. I can spot my own potential pain by a combination of bodily damage and pain killing tablets. I suspect him of the same blunder as Descartes on this one.