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 particle hits a wall, it bounces off. Resetting the model places the particles at new random locations with new random sizes. Playing the model causes it to step repeatedly. Resetting the model causes it to stop playing. Once the play button has been clicked, it can't be clicked again until the model is reset.