The process by which the complex organic molecules in foods are broken down into simpler units which can be utilized by cells for energy or incorporated into living protoplasm. Frequently, there is an essential mechanical pretreatment of the food, either before or after it is taken into the body. Mechanical treatment is especially needed in herbivorous animals that depend on the utilization of cellulose and other plant materials, or where the prey has a hard exoskeleton. Many animals have devices for crushing, grinding, or mixing.