In the classical physics observed in everyday life, matter can be defined as any substance that has mass and takes up space, or as any substance made up of atoms, thus excluding other energy phenomena or waves such as light or sound. Observable physical objects are said to be composed of matter. More generally, in (modern) physics, matter is not a fundamental concept because a universal definition of it is elusive: elementary constituents may not take up space per se, and individually-massless particles may be composed to form objects that have mass (even when at rest).