Analytic results and experiments in ultrahigh vacuum indicate that the static friction between-two clean crystalline surfaces should almost always vanish, yet macroscopic objects always exhibit static friction. A simple and general explanation for-the prevalence of static friction is proposed. "Third bodies," such as small hydrocarbon molecules, adsorb on any surface exposed to air and can arrange to Lock two contacting surfaces together. The resulting static friction is consistent with experimental behavior, including Amontons＇ Laws.