The value of verbal rules given to learners as an instructional aid in forming desired concepts was investigated in this study. Two programs dealing with the concept of conservation of energy in heat exchange between liquids were prepared in the form of self-instructional booklets. One presentation used a picture-rule or expository treatment, and the other used a picture-only or discovery treatment. The programs were used by random halves of nine grade 11 classes (221 students). A 32-item criterion test was used as pretest and posttest. Although no significant differences were found between the two groups either before or after treatment, there was less variation in school means after treatment than before, suggesting a possible ceiling of achievement below the maximum for the test. Discussions with 26 other grade 11 students revealed that the primary cause of errors was a lack of working knowledge of proportions. (MM)