If you pick up a metal ruler, it usually feels cold to your hand. A wood or plastic ruler will feel warmer. But if these rulers were all sitting on your desk, then they are all room temperature. So why does one feel colder than the others? It all comes down to a property of matter called conduction. Conduction  measures how well a material (such as metal, wood, or plastic) transfers energy. The more conductive a material is, the better it transfers energy. For example, good electrical conductors transfer electricity well. Good thermal conductors transfer thermal energy well. In class, you learned about thermal conduction. When two substances with different thermal energies touch, thermal energy is always transferred from the substance with more thermal energy to the substance with less thermal energy until the thermal energy in the two substances is equal. It’s kind of like the metal, glass, and plastic are soaking up as much thermal energy as they can from the water. Metal heats more quickly because it is a better conductor than plastic or glass: it can How will you do this? How will you keep thermal energy from transferring out  of your cooker? Images, right to left:  / Public Domain - publicdomainvectors soak up thermal energy more quickly than the others. But it only does this until it is the same temperature as the water. Let's consider the rulers again. How does conduction help explain why the metal feels colder? Well, your hands are at body temperature, about 37°C (98.6°F). They are kept warm by the blood in your veins and have more thermal energy than the rulers. The rulers are at room temperature, which is probably between 20 and 30°C. When you touch each ruler, they begin absorbing thermal energy from your hand. Since the metal is a better conductor, it will take your thermal energy more quickly than the plastic or wood. The experience of losing thermal energy makes your hand feel “cold”. What if instead of a metal ruler, you were touching a big metal table? It would feel cold for much longer because there is so much more metal. It would take much more thermal energy to warm up to the same temperature as your body, meaning it would transfer a lot more thermal energy from your hand. This would make your hand feel quite chilly! How long it takes an object to change temperature depends on both the number of molecules in a substance and the thermal conductivity of the material it is made of. In your solar cooker, you will want to transfer as much thermal energy as possible to the food.