The lexical semantic structures of change-of-state verbs are explored via linguistic theory, corpus analysis, and psycholinguistic experimentation. The data support the idea that these verbs can be divided into two classes, those for which the change of state is internally caused and those for which it is externally caused (Levin & Rappaport Hovav 1995, cf. Smith 1970). External causation change-of-state verbs have been hypothesized to denote two subevents, internal causation change-of-state verbs only one event. Consistent with this difference, the psycholinguistic data indicate that, in both transitive and intransitive constructions, sentences with external causation verbs take longer to comprehend than sentences with internal causation verbs.