Watch. Practice. Learn almost anything for free. With a library of over 2,800 videos covering everything from arithmetic to physics, finance, and history and 298 practice exercises, we're on a mission to help you learn what you want, when you want, at your own pace. All of the site's resources are available to anyone. It doesn't matter if you are a student, teacher, home-schooler, principal, adult returning to the classroom after 20 years, or a friendly alien just trying to get a leg up in earthly biology. The Khan Academy's materials and resources are available to you completely free of charge at www.khanacademy.org.