Radiometric dating of the most ancient fossils uses the

The result is like a radioactive clock that ticks away as unstable isotopes decay into stable ones.

You can't predict when a specific unstable atom, or parent, will decay into a stable atom, or daughter.

The more parent isotopes there are -- and the fewer daughter isotopes -- the younger the sample.

If you try to add extra blocks to the sides pyramid, they may stay put for a while, but they'll eventually fall away.Usually, atoms have an equal number of protons and neutrons.If there are too many or too few neutrons, the atom is unstable, and it sheds particles until its nucleus reaches a stable state.Geologists have a much harder job keeping track of time.Studying the Earth and its evolution, they work with time scales of thousands to billions of years.

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