Rubidium-strontium (Rb-Sr) dating: Radioactive rubidium-87 decays into strontium-87 with a half -life of 48.8 billion years.Not surprisingly, Ru-Sr dating is used to date very old rocks (as old as the Earth, in fact, since the Earth is "only" around 4.6 billion years old).

Sure, you can scour the Internet and learn rather quickly that the scientific consensus pins the age of of the planet at about 4.6 billion years.

But Google didn't invent this number; instead, human ingenuity and applied physics have provided it.

The half-life of uranium-238 is 4.47 billion years, while that of uranium-235 is 704 million years.

Because these differ by a factor of almost seven (recall that a billion is 1,000 times a million), it proves a "check" to make sure you're calculating the age of the rock or fossil properly, making this among the most precise radiometric dating methods.

U-Pb dating is often used to date igneous (volcanic) rocks, which can be hard to do because of the lack of fossils; metamorphic rocks; and very old rocks.

All of these are hard to date with the other methods described here.

For example, certain drugs, including ethyl alcohol, are metabolized by the body at a fixed number of grams per hour (or whatever units are most convenient).

If someone has the equivalent of five drinks in his system, the body takes five times as long to clear the alcohol as it would if he had one drink in his system.

Many substances, however, both biological and chemical, conform to a different mechanism: In a given time period, half of the substance will disappear in a fixed time no matter how much is present to start with.