With the BallsBounce app, you can place between 1 and 50 balls on the screen and then sit back to watch them bounce off the walls and collide with one another. You have control over the number of balls, the size of the balls, the temperature (this sets their speed – set it to very cold to see the balls in slow motion), and whether the balls collide with one another or not.
In addition, you can choose to use random colors, or set the colors of the balls yourself. Ball 1 (the ball that is drawn on top of the others) can be set to one color, and the remaining balls can be set to a different color, to make it easy to follow one of the balls. The “random colors” toggle switch over-rides the color sliders for the other balls (except the transparency slider), if it is on.
See if you can find the settings that correspond to the pictures shown here, and then adjust the settings to see how things change.
While it is fun just to play with the settings and watch the balls, you can, if you want, use the app to investigate some basic physics and chemistry ideas, including color addition, the behavior of ideal gases, Brownian motion, crystal structure, and phase transitions. If none of that means anything to you, don't worry about it – just have some fun with it.