The purpose of this article is to investigate the effects of instruction of the operational rule on the comprehension of the scientific law of conservation of mass by third-grade children (N=37). In class, the teacher taught the rule: “if there is no transfer of matter, the weight of an object does not change.” Learners were expected to predict whether the weight of an object would change, and to decide whether a transfer of matter would occur by measuring the change in weight. The outcomes of the lessons were analyzed through post-tests, and the teaching-learning process during the lessons was also examined. The results of the post-tests showed that learners could apply the rule to problems similar to that used during the learning session but not to dissimilar problems. The analysis of the teaching-learning process showed that the teacher based the explanation of the rule mainly on inverse and contrapositive statements. The lack of converse statements may have led to a limited understanding of the meaning of “transfer of matter” and failure to apply the rule when problem solving.