The price of a pizza is related to the number of toppings as well as its size.
What type of function (linear, quadratic, or exponential) would you create to describe the cost of the pizza vs. the number of toppings?
How about the cost of the pizza vs. its radius?
If you cannot find a menu from a pizza place, use the Internet pizza server to analyze some data at http://www.ecst.csuchico.edu/~pizza/pizzaweb.html.
Submit your idea for an investigation to InterMath.