Cooking foods to the proper temperature kills harmful bacteria that can cause food-borne illness. Using looks and texture are not good indicators of doneness. The only way to know for certain if a food is thoroughly cooked is to use a food thermometer. To properly use a food thermometer, it should be inserted at least 2 inches into the thickest part of the food without touching fat or bone. For thin foods, like burger patties, it can be inserted sideways.