We conducted two studies of beliefs about laboratory and everyday thermal phenomena. The first study identified concepts of heat energy and temperature held by adolescents, adults, and scientists. We found a classic separation of “school” and “everyday” knowledge in each population. We conducted clinical interviews with 37 middle school students, 9 adults, and 8 chemists and physicists to obtain their predictions and explanations of real-world phenomena. Many students believed that metals “conduct,” “absorb,” “trap,” or “hold” cold better than other materials and that aluminum foil would be better than wool or cotton as a wrapping material to keep cold objects cold. Respondents in each group held many intuitive ideas that were well established. Although scientists made more accurate predictions than students and gave theoretical definitions of terms, they too had difficulty explaining everyday phenomena. The second study investigated the impact of a middle school science curriculum designed to help students understand everyday thermal events. We found marked improvements in posttest scores and clinical interview responses as a result of instruction that built on students' intuitions.