Most electron microscopists appreciate that proper operation of the source is vital to the operation of the entire instrument. But because the gun involves a complex interplay of phenomena, few of which can be directly observed, some common misconceptions have gained currency — typically these "myths" contain a kernel of truth, but they can also mislead when accepted uncritically. In this series of articles, we explore three areas of common misunderstanding relative to operation of the thermionic {tungsten filament or LaB6) gun. (Field emitters are subject to very different considerations.)