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I compare functions to a Coke a machine. I draw a picture of a coke machine on the board with five or six buttons of different sodas. I ask the class if anyone has ever pushed a button for a soda and not gotten the soda they wanted. I then explain that the soda machine isn’t “functioning” if you sometimes get what you want, and sometimes get a different soda. “If I push the Dr. Pepper button, I’d better get a Dr. Pepper EVERY TIME. Otherwise it’s not a function.” I then put a series of ordered pairs on the board and compare the numbers to sodas, challenging the class to decide if it’s a function. (Note: I always include two coke buttons so students see that it’s ok to have two different inputs (x) produce the same output (y))