The smallest unit of a substance that retains its chemical identity, consisting of two or more atoms joined by chemical bonds. A water molecule, H₂O, behaves as water; split it into hydrogen and oxygen and those properties vanish. Molecules range from simple gases to giant polymers like starch and DNA containing millions of atoms. In food, molecules of sugars, proteins, fats, flavours, and colours interact to create taste, texture, and nutrition. Chemistry is essentially the study of how molecules are built, how they change in reactions such as cooking, and how their structures determine their properties.