After all, in the absence of external energy input, random thermal motion tends to destroy order. This tendency is codified in the second law of thermodynamics, which dictates that everything ages and decays: Buildings and roads crumble; ships and rails rust; mountains wash into the sea. Lifeless structures are helpless against the ravages of thermal motion. But life is different: Protein machines constantly heal and renew their cells. In this sense, life pits biology against physics in mortal combat.