Algodoo

This is a harmonograph, which draws things using two pendulums, one decides the x-coordinate of the "pen", and one decides the y-coordinate. First, set the starting point for the pen using the arrow keys (each needs to be at least a little bit off center or else it won't do anything). Then, press the enter key to set the pendulums in motion. You can change the mass of each of the green weights, which will change the speed of the swing. The starting point of the pen will determine the final shape, and the ratio between the weights will change how the center is drawn in. If you want to stop the pendulums, just press enter. If you want to take a screenshot I recommend stopping the sim and selecting everything and moving it out of the way (the drawing will remain), then you can turn off the grid too. The difference between this one and the first one is that this one always draws rectangles. I recommend speeding up the sim as it can take a while to finish drawing.