Some middle-school students have difficulty explaining the process of heating and cooling in terms of heat being transferred. Some think that "cold" is being transferred from a colder to a warmer object, others think that both "heat" and "cold" are transferred at the same time. Both middle- and high-school students have difficulty explaining heat-exchange phenomena as interactions. For example, some students think objects cool down or release heat spontaneously—that is, without being in contact with a cooler object. Even after instruction, students find it hard to give up their naive notion that some substances (for example, flour, sugar, or air) cannot heat up or that metals get hot quickly because "they attract heat," "suck heat in," or "hold heat well." Middle-school students even believe different materials in the same surroundings have different temperatures if they feel different (for example, metal feels colder than wood). As a result, students fail to recognize the universal tendency to temperature equalization. Few students (middle- and high-school) understand the molecular basis of heat transfer even after instruction. Although specially designed instruction as opposed to traditional instruction appears to give students a better understanding about heat transfer, some difficulties often remain. (Benchmarks for Science Literacy, pp. 337-338.)