A lump of uranium left to itself will gradually decay, one nucleus at a time. The rate of decay is measured by how long it would take for half of a given bunch of uranium atoms to decay (the half-life). The decay of an individual uranium nucleus is completely unpredictable, but we can accurately predict the way a large lump of uranium will decay.

It is upsetting to think that chance can rule physical properties. In response to this theory Einstein proclaimed "God doesn't play dice!" (Einstein was wrong.)