There are three axioms of probability: (1) Chances are always at least zero. (2) The chance that something happens is 100%. (3) If two events cannot both occur at the same time (if they are disjoint or mutually exclusive), the chance that either one occurs is the sum of the chances that each occurs. For example, consider an experiment that consists of tossing a coin once. The first axiom says that the chance that the coin lands heads, for instance, must be at least zero. The second axiom says that the chance that the coin either lands heads or lands tails or lands on its edge or doesn't land at all is 100%. The third axiom says that the chance that the coin either lands heads or lands tails is the sum of the chance that the coin lands heads and the chance that the coin lands tails, because both cannot occur in the same coin toss. All other mathematical facts about probability can be derived from these three axioms. For example, it is true that the chance that an event does not occur is (100% − the chance that the event occurs). This is a consequence of the second and third axioms.