In this text excerpted from chapter 3, Struggle for Existence, of Charles Darwin's On the Origin of Species, Darwin draws on firsthand and historical information for his observations about evolution.

Experiment with conductivity in metals, plastics and photoconductors. See why metals conduct and plastics don't, and why some materials conduct only when you shine a flashlight on them.

The levels of contaminants found in particular animals vary widely depending on where they fit into the Arctic food chain, as described in this video segment adapted from LOKE Films and the Arctic Monitoring and Assessment Programme.

In this video segment adapted from LOKE Films and the Arctic Monitoring and Assessment Programme, learn how human populations in the Arctic are affected by industrial contaminants in the food chain.

In this lesson designed to enhance literacy skills, students learn how the theory that explains the position of Earth's continents was established and later modified, and gain important insights into how science and the scientific community operate.

This video segment adapted from the Atmospheric Radiation Program explains the differences in the formation of tropical convective cloud systems over islands and over the ocean.

In this video segment adapted from ZOOM, two solar cookers are tested against a control to see which can cook a "s'more" faster.

In this video segment, ZOOM guest Cassie takes us on a tour of the coral reef near her home in Key Largo, Florida, and points out some of its unique features.

This video from NASA features the Cosmic Origin Spectrograph (COS), which allows scientists to use spectrographic analysis to assess the composition of intergalactic material.

In this video excerpt from NOVA, find out how whole genome sequencing saved the life of Alexis, a fraternal twin who was originally diagnosed with cerebral palsy but, in fact, had an even rarer genetic condition.

This interactive activity from ChemThink takes a closer look at a covalent bond--how it is formed and how the sharing of two electrons can keep atoms together.

In this video segment from Nature, learn the geological process by which the next Hawaiian island will be formed.

Students learn how DNA fingerprinting has been used in criminal investigations and discuss the limitations of and problems with DNA testing.

In this interactive activity from the Building Big Web site, investigate dams in distress and decide if they should be repaired, removed, or left alone.

This video segment from Evolution: "Darwin's Dangerous Idea," together with interviews with Daniel Dennett and James Moore, depicts Darwin struggling with publicizing his revolutionary theory.

In this video segment adapted from NOVA, scientists search for carbonized remains of plants preserved in lava flows to find out how long it has taken rain forests on Hawaii to regenerate after a volcanic eruption.

Without the work of decomposers, living organisms would eventually use up all the raw materials in the environment, and dead organisms and wastes would pile up. This video segment from Interactive NOVA: "Earth" describes the role of decomposers as the earth's great recyclers.

In this video segment adapted from ZOOM, cast members use a hair dryer to balance a ball in a stream of air, seemingly defying gravity.

In this video segment, the ZOOM cast is challenged to keep a ping pong ball in a funnel while the funnel is held upside down, seemingly defying gravity.

Find out how controlled explosions are used to demolish multi-story buildings in this interview from the NOVA Web site.