Energy is defined in mechanics as the capacity to do mechanical work, measured as the product of force and distance. A small wave hitting against the shore is energy moving, not an actual object. Metabolism is defined as the set of processes by which cells and organisms acquire, rearrange and void commodities in ways that sustain life. Energy metabolism is defined as the set of processes by which organisms acquire energy (or the capacity to do work), channel energy into useful functions, and dissipate energy from their bodies. There are two categories of energy metabolism: catabolic processes (which break down organic molecules and release energy) and anabolic processes (which build organic molecules and use energy). The 1st law of thermodynamics states that energy cannot be created or destroyed it can only change forms. Energy is always conserved and escapes (flows) as waves in the form of heat and sound.