John Green explains how historians have determined the age of the Earth. He mentions that they created a quantitative chronology of human history by using ancient documents and events. He also states that various thinkers throughout history have used fossils and rocks to determine the Earth's actual age.

This clip illustrates many of the important events in the story of life on Earth. It also explains the Geologic Time Scale, which is the system used to determine the Earth's age.

The Secret Labbers learn about gemstones from Sheila. She also gives examples of the real-world applications of minerals, such as diamonds. Watch Thomas Edison's Secret Lab on Netflix, the Kid Genius channel on Xfinity, public television and DVD. Visit EDISONSECRETLAB.COM for games, videos, downloads and more!

Beakman explains how caves are formed by acid dissolving limestone and he uses a model to show how this happens. He explains there are two major sections of a cave called the twilight zone (little light reaches in) and the dark zone (no light reaches). Beakman describes the different animals that live there and their features. They also talk about stalagmites and stalagtites and then make their own.

See the awesome destructive power of landslides.