Full Idea
What makes Goodman's example a puzzle is the dubious scientific standing of a general notion of similarity, or of kind.
Gist of Idea
Grue is a puzzle because the notions of similarity and kind are dubious in science
Source
Willard Quine (Natural Kinds [1969], p.116)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.116
A Reaction
Illuminating. It might be best expressed as revealing a problem with sortal terms, as employed by Geach, or by Wiggins. Grue is a bit silly, but sortals are subject to convention and culture. 'Natural' properties seem needed.