Consider a mixture of a rapidly decaying element A, with a half-life of 1 second, and a slowly decaying element B, with a half-life of 1 year.

But on the second day, there is no reason to expect that one-quarter of the puddle will remain; in fact, it will probably be much less than that.

This is an example where the half-life reduces as time goes on.

(In other non-exponential decays, it can increase instead.) The decay of a mixture of two or more materials which each decay exponentially, but with different half-lives, is not exponential.

After another 5,730 years, one-quarter of the original will remain.

On the other hand, the time it will take a puddle to half-evaporate depends on how deep the puddle is.

A half-life usually describes the decay of discrete entities, such as radioactive atoms.

In that case, it does not work to use the definition that states "half-life is the time required for exactly half of the entities to decay".

There are various simple exercises that demonstrate probabilistic decay, for example involving flipping coins or running a statistical computer program.