That's a good question. There are tiny bits of radioactive chemical elements in rocks and fossils. Scientists can tell how old the rocks and fossils are by seeing how much radioactivity is left. Radioactivity has a half-life - that's the time for half of the radioactivity to decay [disappear]. For radioactive carbon - C-14 - the half life is about 6,000 yrs, so after 6,000 yrs, there's half as much C-14, and after about 12,000 yrs, there's a quarter as much C-14. C-14 is good for finding the age of things that are less than about 70,000 yrs.

Some radioactive elements have much longer half-lives, so they're good for dating things up to millions or even a billion yrs old.

Another way to date fossils is to look at the layer of rock where they're found Older rock layers get buried by younger rock layers, so the rock layer on the bottom is the oldest, and the rock layer on the top is the youngest.

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