One of the primary objectives of the introductory physics laboratory is to teach error analysis. We describe a very simple and inexpensive experiment which exposes students to the central ideas of estimation and uncertainty, and to the evaluation of theory by graphical display of data. The task is to measure the diameter of a crater formed by dropping a small steel ball into a sand-filled container, and then to deduce the functional dependence of the crater diameter on the kinetic energy of the falling ball.