Although there exists large real-world shapes and structures made from Lego, the arrangements of the Lego pieces in these shapes are often designed manually. In this paper, we solve the problem of taking a 3D model as input, and generating an arrangement of individual Lego pieces that can be assembled together to form a physical object resembling the input model. We define a novel Lego rewriting system that is both grammar-based and data-based. Our Lego rewriting system describes how Lego pieces can be connected to each other to form larger shapes, and we use this system to analyze 3D models to solve our problem. We demonstrate our approach with 3D models from the Princeton Shape Benchmark and Trimble 3D Warehouse.