We introduce a novel mechanism for creating line drawings from three-dimensional models, which captures the dynamic nature of the drawing process. The approach takes into account the interaction between the moving human hand and the drawing instrument. This is demonstrated as applied to the specific problem of making silhouette drawings from polygonal models. A control system drives a pen by tracking the contour of the polygonal model as projected onto the drawing surface, thus mimicking hand motion. The pen is treated as a physically-based object with momentum, giving the generated lines a smooth hand-drawn quality. Lines are rendered using a ribbon metaphor, where thickness is determined by the twist of the ribbon. The twist angle can be dependent upon various attributes such as perspective depth, the curvature of the line, and the lighting of the model. A number of examples are presented, ranging from tightly controlled drawings to expressive gestural drawings.