Two experiments investigated kindergarten through fourth-graders' and adults' ability to evaluate the certainty of deductive inferences, inductive inferences, and guesses, and explain the origins of inferential knowledge. Findings indicated that children rated their own deductions as more certain than guesses, but when judging another person's knowledge, did not distinguish valid from invalid inferences and guesses until Grade 4. By Grade 3, children differentiated their own deductions from inductions and guesses. (JPB)