Stretching a rubber band is a physical change because only its shape and size change, and its chemical composition remains the same. It can also return to its original shape. Cooking an egg involves the denaturation of proteins, forming new substances. Burning a candle (the burning of wax) produces new substances like carbon dioxide and water vapor. Digestion of food breaks down complex substances into simpler ones through chemical reactions. All these are examples of chemical changes as new substances are formed.