In the example of the roasting chicken, let z denote the time it takes for a chicken of mass m to cook at a temperature of T degrees. Clearly, z is dependent on m and T. Thus, there is a relationship between m, T, and z. In other words, there is a rule, or function, that associates to each ordered pair (m,T) a cooking time z. In the notation of mathematics, we write this relationship as z = f(m,T) where f is the rule that associates the ordered pairs with the cooking times. This function can be represented either numerically, algebraically, or graphically.