Sal Khan is a math, science, and history teacher to millions of students, yet none have ever seen his face. Khan is the voice and brains behind Khan Academy, a free online tutoring site that may have gotten your kid out of an algebra bind with its educational how-to videos. Now Khan Academy is going global. Backed by Google, Gates, and other Internet powerhouses, Sal Khan wants to change education worldwide, and his approach is already being tested in some American schools. Sanjay Gupta reports.