Two-Factor Authentication: Circa 1944
To help explain two-factor authentication to my customers, I typically phrase it as "something you have, and something you know." This is often seen at high-security places such as airports, where to get to a secure location, an employee must swipe a badge (something they have) and then type in a code (something they know). The following video illustrates this perfectly - To gain access to Hogan's spy network, the German officer (also an American spy) must authenticate himself to Hogan using something he has (half of a playing card) and something he knows (half of a passphrase):