About
Our model should have a world with multiple randomly-placed, randomly-sized, stationary particles. There are buttons to play, step and reset the model. Stepping the model causes a particle to move if the particles hit a wall, it bounces off. Resetting the model places the particles at a new random location 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 cannot be clicked again until the model is reset.