We investigated the roles of comparison and explanation in teaching children an important engineering principle – that triangular cross-bracing confers stability to structures. We aimed to discover how best to convey this principle to 4- and 6-year-olds and to reveal the cognitive mechanisms involved. Children either compared contrastive cases (a braced building vs. a non-braced building), received an explanation of the principle, or both, and were then tested on their ability to apply the principle to various contexts. We found that 4-year-olds benefited from comparison, but surprisingly did not benefit from a combination of comparison and explanation. 6-year-olds, however, benefited greatly from the combination, suggesting that more developed abilities are required to combine the two inputs. Performance on a mental transformation task was also related to successful brace placement. These findings suggest that comparison and explanation can both contribute to learning, both singly and together, depending on ability and/or age.