The demos demonstrate iterations of the bouncing ball.
I thought it would be fun to see the progression
Demo1 is a cube that is copied directly from the README text of Three.js.
In demo2 the basic design is the same but the cube is changed to a few spheres (balls) bouncing around an invisible room.
Demo3 improves upon this by making the walls visible whenever they are hit.
Demo4 and demo5 add animation and opacity to the walls as well as making the view smaller so it looks better in the browser.
Demo5 further adds a light source to the room.
Demo6 adds a sound when the ball hits the walls.
Demo7 adds movement with the arrow keys or wasd.