Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: https://www.khanacademy.org/science/cosmology-and-astronomy/earth-history-topic/plate-techtonics/v/structure-of-the-earth Structure of the Earth - crust, mantle, core More free lessons at: http://www.khanacademy.org/video?v=4AxZ-6MOznY