Massive Open Online Courses (MOOCs) are becoming the textbooks of the 21st century. I describe what a MOOC is, and try to answer the questions: Why make a MOOC? Who are MOOCs for? and How to make a MOOC? Anxiety about MOOCs replacing teachers is largely misplaced. I describe my on-going experience of putting together an astrobiology MOOC for the Australian National University.