On the basis of responses to written questions administered to more than one thousand introductory chemistry students, we claim that students often rotely apply memorized combustion rules instead of reasoning based on explanatory models for what happens at the molecular level during chemical reactions. In particular, many students argue that combustion produces carbon dioxide and/or water, even when the reactants do not contain hydrogen or carbon, an answer that is inconsistent with the principle of atom conservation. Our study also corroborates the finding that students frequently say that oxygen is "necessary for" or "used in" combustion reactions without connecting this reasoning to conservation principles, suggesting that this likewise may be a rotely applied, memorized rule.