Functions describe the relationships  between members of two sets of numbers. They are used to model all types of change or variation in real world settings. For example, arm length is a function of height. The length of a person's arm is related to her/his height; circumference is a function of diameter; the bigger the diameter the greater the circumference of a circle,  Functions can be expressed using words, graphs, equations and tables. 

1. The best Function Machine in the world
   Worksheet for use with this function machine.

2. Another OK function machine

3. Another pretty good function machine

4. A more difficult function machine