The third side of a coin is essentially the one that connects the other two sides: the head and the tail. This analogy is used throughout the book to emphasize the importance of "mastery learning" in any profession, such as medicine, law, or engineering. Mastery learning is the essential outcome of uniting the two flat faces of a coin with the help of the third circular side. In any profession, the two flat faces of the coin are theory and practice, and the third side is the testing. The author gives examples from his more than fifty years of experience in engineering practice and engineering teaching to prove that mastery learning is essential. In the very rapidly changing pace of technology today, any curriculum that ignores mastery learning is bound to produce obsolete engineers.