Two experiments assessed effects of activation of prior knowledge through small-group discussion. Subjects were given a description of natural phenomena and were asked to elaborate on possible explanations for them. In Experiment 1, small groups of subjects were presented with a problem describing the behavior of a blood cell in pure water and in a salt solution. No additional text was studied. The experimental subjects produced more than twice as many propositions about osmosis (i.e. the biological process explaining the blood cell's behavior) as a control group produced. Experiment 2 investigated effects of problem analysis on subsequent text processing for subjects with imprecise prior knowledge (novices) and subjects with precise knowledge (experts). Recall of the text showed considerable facilitative effects of problem analysis. Results are explained in terms of faster accessibility of prior knowledge and better integration of new information into explanatory that may exist before, or are actively constructed during, problem analysis. Attempts to understand the physical world involve the use of cognitive structures that represent mechanisms or principles underlying the phenomena observed. These cognitions may vary from highly sophisticated to quite naive. They may emerge as the result of formal education but often are constructed spontaneously while a person is experiencing the phenomena concerned. Take thunder and lightning. By scientists, this natural phenomenon is interpreted in terms of large differences in electrical potential between clouds charged with static electricity and the earth. Young children however, generally favor explanations involving the clash of clouds, and in earlier centuries thunder was attributed to the rage of the gods. These naive mental models are, like the of science, not merely descriptive of what is going on in the outer world. They are truly explanatory, because they clarify why the world is as it is. In addition, these conceptions can be considered because they usually consist of a set of concepts connected by causal links that help to interpret the phenomena concerned in terms of the underlying structure of these phenomena (Clement, 1979; Gentner & Stevens, 1983).