A linear function is a rule y = mx + b whose graph is a straight line: m is the rate of change and b is the initial value. Two functions can be compared whatever form they come in, a table, a graph, a rule or a description, by finding each rate of change. Not every function is linear: the area of a square bends. The crew times a dripping jug at the Puzzle Post, reads rates and starts from tables and points, and describes graphs in words.