How can heat cooks food? My classmate asked me this but I didn't imagine how it works.
A great deal of food consists of mixtures of long-chain carbohydrates (starchy stuff) and proteins. Some of these things are hard for the human digestive system to process (try getting any calorific value from a raw potato!).
Cooking causes substantial chemical changes to those not-very-digestible components. Proteins are denatured by heat and made easier to digest; carbohydrates are broken up to some extent and smaller, easier to digest shorter chain carbohydrates are formed. In addition, the structural components in some foods are degraded making it easier to get the digestible components.