Daniel has been watching a dinosaur documentary. The narrator says a fossil is sixty-five million years old.

He never stopped to wonder how anyone actually knows that.

The fossil does not come with a date on it. And scientists usually do not date the fossil itself -- they date the rock around it. This episode explains how, starting with one of the most elegant ideas in all of science: that certain atoms decay at such a perfectly predictable rate that they function as natural clocks, running continuously since before anyone was there to read them.

What you will find in this episode:

• What radioactive decay actually is -- and why it happens at a fixed, predictable rate
• What a half-life is and how measuring it gives you a date
• Why carbon-14 dating does not work for dinosaurs -- and what does instead
 ...