In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.
But for the sprawling sphere we call home, age is a much trickier matter.
Previously she was an assistant editor at Scholastic's Science World magazine.
Jeanna has an English degree from Salisbury University, a Master's degree in biogeochemistry and environmental sciences from the University of Maryland, and a science journalism degree from New York University.
Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.
All these methods point to Earth being very, very old -- several billions of years old.
Some, however, are unstable -- given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element.