Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.

When asked for your age, it's likely you won't slip (with the exception of a recent birthday mistake).

But for the sprawling sphere we call home, age is a much trickier matter.

Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.

The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.

These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.

Our planet was pegged at a youthful few thousand years old by Bible readers (by counting all the "begats" since Adam) as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of 100 million years.

Kelvin defended this calculation throughout his life, even disputing Darwin's explanations of evolution as impossible in that time period.

Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth is difficult to determine.

It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.

Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.