Four experiments tested three competing theories of metaphor comprehension: comparison, categorization, and career-of-metaphor. The findings shed light on key mechanisms involved in metaphor processing and conceptual combination. They show that some novel tropes are privileged in metaphor over simile form, and others may express different interpretations in simile and in metaphor form. These results speak against the assumption that metaphors and similes are interchangeable, thus providing support for the categorization model. A unifying account of metaphor comprehension is proposed, along with (...) a discussion of implications for conceptual combination. (shrink)