Geometry starts with three ideas it does not define: a point, a line, and distance (along a line or around a circular arc). From those, every other word gets a precise definition: a segment, an angle, a circle, perpendicular lines and parallel lines. With only a compass (string and chalk) and a straightedge, the crew copies segments and angles, bisects them, builds perpendicular and parallel lines, and inscribes an equilateral triangle, a square and a regular hexagon in the turntable circle. Every step of a construction has a reason.