Communication occurs when there needs to be a movement of information from one place to another. You have it here, but you want it there. Communication involves a sender, or source of the information, a receiver located elsewhere, and a channel that carries the information from sender to receiver. When one person talks to another, the channel consists of soundwaves carried through the air between them. Errors frequently occur, because of interference or “noise” in the channel, because the sender and receiver do not share the same code, or because of physical limitations at either end. A basic method for avoiding or at least detecting errors is to use “redundancy” – extra information added to the message for this purpose. For example, If I tell you my phone number and then repeat it, I am adding redundancy. Other forms of redundancy are built into message itself, such as city, state and zip code on a letter. Communication involving a computer also use redundancy. The simplest form is parity: one or more bits added to a message to guarantee an even number of 1’s. In the unit, the instructor plays a “magic trick” on the class, using parity to determine where a counter was flipped in a 5 x 5 array. The unit then looks at how another form of redundancy, the check digit, detects errors in scanning supermarket bar codes. Students compute the check digit on a bar code and then look to see if they were right.