Children's understanding of the epistemological terms "know" and "guess" was investigated in two studies with four- to ten-year-old subjects. Two adult players guessed at the location of a ball hidden in one of two boxes. On each trial the child was asked questions about "knowing" and "guessing" both before and after the guessing took place. Questions using "know" were asked separately from those using "guess," to avoid the assumption of semantic oppositeness of these terms, an assumption which previous investigations have made. Responses were analyzed in relation to the pattern of correct/incorrect guesses on each trial, and whether or not one player actually observed the ball being hidden prior to guessing. Results indicated that younger children shifted between two acquired senses of "know" during the experiment: the sense of subjective certainty due to prior information, and the sense of correct conjecture, as determined by outcome. Further, younger children did not treat "know" and "guess" as opposites, but instead attribute both epistemological states to the correct player; later, "knowing" is attributed to the correct player, and "guessing" to the incorrect player. Not until age 9 or ten do children appear to be completely independent of outcome in their response to both questions. (Author)