Something you know- authentication is accomplished through something you know, something you have and/or something you are. the canonical example of something you know is a password or pass phrase. you might type or speak the value. a number of schemes are possible for obtaining what you know. it might be assigned to you, or you may have picked the value yourself. constraints may exist regarding the form the value can take, or the alphabet from which you are allowed to construct the value might be limited to letters only. if you forget the value, you may not be able to authenticate yourself to the system. something you have, would be a physical item you possess, such as a smartcard. something you are, would be a personal characteristic of you, not a piece of information you know.