pt. I. Where scientific theories come from. The nature of an explanation -- An example of a scientific theory: The motion of the planets -- The speed of light -- pt. II. The idea of energy. Energy: A basic law of physics -- More about energy: Using the idea -- Wave motion and sound -- Heat energy -- Chemical energy: Atoms and photosynthesis -- Heat engines: The second law -- pt. III. The electromagnetic spectrum. Electromagnetic waves: From radio to infrared -- Visible light and ultraviolet: Waves and particles -- X-rays, gamma rays, and E=MC² -- pt. IV. The frontiers of science. Nuclear energy -- Beyond the solar system.