The turtle carries a pen that can draw pictures. At any time the pen can be "up" (in which case moving the turtle does not change what's on the graphics screen) or "down" (in which case the turtle leaves a trace). If the pen is down, it can operate in one of three modes: "paint" (so that it draws lines when the turtle moves), "erase" (so that it erases any lines that might have been drawn on or through that path earlier), or "reverse" (so that it inverts the color of each point along the turtle's path).