Given that the use of mental verbs is linked to the acquisition of the theory of mind (TM), the objectives of this investigation are: A) to study the organization of such verbs in clusters of 14 mental verbs of knowing and B) to design a verb extension task to analyze the use of such activities. Classifi cation of mental verbs was carried out by 63 experts and the extension task was completed by 135 university students. The conclusions are: the experts organize the verbs following the development of the TM in the usual clusters of Input, Memory and Processing; they do not distinguish between Constructive Processing and Non-constructive Processing and they present an additional cluster named General. In addition, the extension task allows us to analyze the development of the TM by studying the organization of verbs of knowing.