This article reports on a study that investigated novice students' rules for determining which of two objects exerted the greater force on the other. Subjects worked five paper-and-pencil task sets depicting different objects interacting with each other. For each situation they were asked to identify which of the two objects was exerting the stronger force on the other. Each subject worked the task sets under one of three conditions: with no aid, with a written statement describing forces as interactions that always occur in pairs, or with a written statement of Newton's third law. The vast majority of the subjects in the study used procedures (rules) which could be identified. There were significant differences in the patterns of usage of the various rules between those who received no written statement and those who received the statement of Newton's third law. The primary difference between the two groups was in the usage of a rule (All Equal) which produced the same response sequence as Newton's third law. However, only 29% of those who received the Newton's third law handout used the All Equal rule on all five task sets. There were no significant differences in usage of the All Equal rule between those who had had high school physics and those who had not. Strong evidence was found to contradict the idea that the subjects who failed to used the All Equal rule simply did not read the written statement properly. The results of this study have implications for learning from text.