Cooking is the application of heat to a food item or items. Cooking, however does much more than just apply heat to food. Because heat applied to food items those changes internally according to the type of heat applied, how much heat applied and for how long.
Sugars and Starches
For instance, sugars in food turn brown or caramelize when heat is applied. Sugars in flour turn brown when the bread reaches close to done.
Starches will absorb water like sponges in the example of cooking rice and the rice absorbs all of the water you’ve measured out prior to cooking.
Proteins will gain structure or get firm when heat is applied. A grilled steak cooked well done steak is firm to the touch while a rare steak will give when pressed with a finger.
Application of Heat
The application of heat to a food item that contains water will evaporate some of that water. Loss of water content causes food to shrink in size.
Fats and Oils
Fats, butters and oils will liquefy when heated and eventually smoke and burn if too much heat is applied.
Fibers in vegetables soften when cooked so celery and carrots actually soften when cooked.
The color of food is affected by the application of heat. Have you ever sautéed Swiss chard which turns a bright green when cooked properly only to yield to a wilted structure and a brown color if cooked too long.
Vitamins and minerals are cooked out of food the longer a food item is heated. That is why steaming vegetables is a more nutritious way of preparing vegetables than parboiling.
Anyone can apply heat to food for human consumption. The art and skill of cooking comes in play when you know what type of heat to apply, for how long and what it will do to the item or items being cooked.