Adopting a constructionally-oriented analysis, the present paper examines the pattern ‘think again’ (i.e., an instance of a mental state verb + adverbial adjunct) in synchronic, corpus-derived data. On the basis of both qualitative and quantitative analyses we show that think again merits constructional status in language; while it inherits features of fully-compositional meaning from its constituents it has also developed its own idiosyncratic properties. We further argue that think again may ultimately function as a discourse marker of challenge that regulates the relationship between Speaker (S) and Addressee (A), correlating with certain contextual regularities and interdependencies. It thus qualifies as a discourse construction that imposes a dialogic construal on its context and contributes fundamentally to discourse unit delimitation.