You can learn anything. For free. Spend an afternoon brushing up on statistics. Discover how the Krebs cycle works. Get a head start on next semester's geometry. Prepare for upcoming exams. Or, if you're feeling particularly adventurous, learn how fire-stick farming changes the landscape of Australia. Whether you're a student, teacher, homeschooler, principal, adult returning to the classroom after 20 years, or a friendly alien trying to get a leg up in earthly biology — Khan Academy's persona...