About
There is a world with multiple randomly-placed, randomly-sized, stationary particles. There are buttons to play, step, and reset the model. Stepping the model causes the particles to move. If the particles hits a wall, it bounces off. Resetting the model places the particles at a new random location with a new random size. Playing the model causes it to step repeatedly. Resetting the model causes it to stop playing. Once the play button has been clicked, it cannot be clicked again until the model is reset.