A study compared students' cognitive word knowledge of the cognates of "think" and "know" within a theoretical framework focused on hierarchical levels of meaning. Subjects were 31 fifth, 32 seventh, and 21 tenth graders attending single-gender private schools in the Washington, D.C. metropolitan area, and 70 college undergraduate students in an introductory psychology course at the University of Maryland. Cognitive words form a category within the internal state lexicon and may be central to accessing, monitoring, and transforming internal states, all of which seem to be processes critical to reading comprehension. Cognitive word knowledge was positively correlated with achievement scores. The correlations with cognitive word knowledge were higher for verbal (vocabulary and reading comprehension) than quantitative achievement scores, and cognitive word knowledge increased with age. However, the order of acquisition of cognitive words depended on a complex interaction between the frequency of the cognitive word in established word frequency counts, the level of meaning as determined by the conceptual difficulty hierarchy, and whether the cognitive word was a cognate of "think" or "know." (Contains 61 references, 6 tables, and 3 figures of data. The cognitive word task is attached.) (Author/RS)