The most simple function of all is an extremely straight forward one, it's called the constant function.
An example is the great American Institution of the fixed price all you can eat buffet.
Recall that a function is a rule that describes how an output depends on an input. Here the input is the quantity you eat and the output of the ``cost function'' is the price you pay for it. Of course, by definition there is NO relationship between quantity eaten (x-variable) and the price (y-variable), that is, the amount you pay does not depend on how much you eat.
