Cesium 137 dating wiki
As an example, the radioactive decay of carbon-14 is exponential with a half-life of 5,730 years.
On the other hand, the time it will take a puddle to half-evaporate depends on how deep the puddle is.
The number at the top is how many half-lives have elapsed.
Note the consequence of the law of large numbers: with more atoms, the overall decay is more regular and more predictable.
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.