What we learned from creating one of the world's most popular MOOCs.

(LHTL) is currently one of the world's most popular massive open online course (MOOC), with nearly 2.5 million registered learners in its first 4 years. Here, we "reverse engineer" the design of the course's videos to show how creative application of well-known principles of multimedia learning in an MOOC context appear to have fueled the course's popularity. Gaps in knowledge of multimedia learning are also noted. There have been some 50 years of experience researching effective classroom teaching, but less there have been only 5 years since MOOCs became widespread. The success of LHTL may provide further insight into the importance of the principles of multimedia learning, and how those principles might be practically implemented to improve MOOC making and the general design of instructional videos.