So far you have mostly worked with equations. There are special types of equations, called functions, that have exactly one output for every possible input into the equation. An example of this in the real world is the amount of money you are charged at a store: the input is the item you are purchasing, and the output is the money you are charged. Individual items have one price, so this is an example of a function. Can you think of any other examples of functions? Why might this type of equation be useful?