Teaching the concept of a function. I am doing a class for at risk high school math students on the concept of a function.  I have seen all the Internet lesson plans and different differentiated instruction plans.  The idea of a function as a machine has always sat well with me, so I was thinking of playing off that.  Are there any "out of the box" ideas that perhaps someone used or saw or knows that might hit home?

Some everyday concepts could help. Such as
In a restaurant menu (f=food item, p=price of item):
Is f a function of p? Is p a function of f?
On back of a mailed envelop (s=street address, z=5-digit zip code): 
Is s a function of z? Is z a function of s?
In a teacher's grade book (n=name of student who took a test, g= grade of student)
Is n a function of g? Is g a function of n?
