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Some elements are unstable, decaying via the weak interaction into other elements.
Such substances are called radioactive . The tendency of an element to decay is expressed by its half-life . If we wait a further 10 minutes one half of the remaining sample of atoms will have decayed and one half will not. After a further 10 minutes one half of that remaining sample will have decayed. This is sort of reminiscent of a famous paradox by Xeno, who argued that we can never walk out of a room.