We illustrate a simple algorithm for approximating the medial axis of a 2D shape with smooth boundary from a sample of this boundary. The algorithm is compared to a more general approximation method that builds on the same idea, namely, to approximate the shape by a union of balls. While not as general, our algorithm is simpler, faster and numerically more stable. Both algorithms are visualized using the Mesecina tool, which is also described. Copyright 2007 ACM.