Conventionality and aptness are two dimensions of metaphorical sentences thought to play an important role in determining how quick and easy it is to process a metaphor. Conventionality reflects the familiarity of a metaphor whereas aptness reflects the degree to which a metaphor vehicle captures important features of a metaphor topic. In recent years it has become clear that operationalizing these two constructs is not as simple as asking naïve raters for subjective judgments. It has been found that ratings of (...) aptness and conventionality are highly correlated, which has led some researchers to pursue alternative methods for measuring the constructs. Here, in four experiments, we explore the underlying reasons for the high correlation in ratings of aptness and conventionality, and question the construct validity of various methods for measuring the two dimensions. We find that manipulating the processing fluency of a metaphorical sentence by means of familiarization to similar senses of the metaphor (“in vivo conventionalization”) influences ratings of the sentence's aptness. This misattribution may help explain why subjective ratings of aptness and conventionality are highly correlated. In addition, we find other reasons to question the construct validity of conventionality and aptness measures: for instance, we find that conventionality is context dependent and thus not attributable to a metaphor vehicle alone, and we find that ratings of aptness take more into account than they should. (shrink)