The format may vary. It might be a video of a teacher speaking to the camera, photographs and text about the topic or some mixture of these. The Khan Academy has been successful in teaching mathematics using notes written using Yahoo!'s doodle pad with a voiceover by Salman Khan (educator). These were then loaded onto YouTube where they have been popular and influential. Such lectures are a key part of flip teaching in which the initial work of communicating the essentials of the topic is done by the video lesson.