# Life dating Thus, the mixture taken as a whole will not decay by halves.

A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity.    Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation.There are various simple exercises that demonstrate probabilistic decay, for example involving flipping coins or running a statistical computer program.The decay of many physical quantities is not exponential—for example, the evaporation of water from a puddle, or (often) the chemical reaction of a molecule.For example, if there is just one radioactive atom, and its half-life is one second, there will not be "half of an atom" left after one second.Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average".