Preparing food with heat or fire is an activity unique to human beings thus being termed as cooking. There is no clear evidence as to when cooking was invented, but most anthropologists believe that cooking fires first developed around 250,000 years ago. Cooks select and combine ingredients using a wide range of tools, techniques and methods. Most ingredients in cooking are derived from living things. Vegetables, fruits, grains and nuts come from plants, while meat, eggs, and dairy products come from animals. Cooking often involves water which is frequently present as other liquids, both added in order to immerse the substances being cooked. There are different methods of cooking, they include baking, roasting, sauteing, stewing, frying, grilling, barbecuing, smoking, boiling, steaming and braising and a most recent innovation is the micro wave oven. When heat is used in the preparation of food, it can kill or inactivate potentially harmful organisms including bacteria and viruses, the effect will depend on temperature, cooking time, and technique used. Although cooking has traditionally been a process carried out informally at home or around a communal fire, cooking is often, and increasingly, carried out outside the home. Thus, "Home-cooking" may be associated with comfort food.