Neil deGrasse Tyson's online lecture course, The Inexplicable Universe, drops from $90 to $10 when you sign up during the Learn Fest promotion at Udemy. That's the best price we can find anywhere. The course, which was made by The Great Courses, includes three hours of on-demand video in the form of six lectures. Topics include string theory, quantum foam, and neutrinos. Professor Tyson discusses scientific research surrounding the subjects and elaborates on the remaining mysteries surrounding each of them. Students have lifetime access to this material upon signup. There's no sales tax.