There is a missing link between our understanding of teaching as high-level social phenomenon and teaching as a physiological phenomenon of brain activity. We suggest that the science of human interaction is the missing link. Using over one-million days of human-behavior data, we have discovered that "collective activenes" (CA), which indicates the simple high-frequency-motion ratio of a group to total time, plays a fundamental role. Even solo-work performance, such as telephone-sales success rate, is more influenced by CA than by one's individual skill level, which has been the conventional target of employee efficiency education. CA is experimentally found to drive people collectively to challenge for greater performance and happiness through a synchronized proactive mind. This is, in fact, deeply related to understanding the question "What is teaching?" (Contains 6 figures.)