(15 minutes or less)
Paint one of the balls a bright color, using latex paint (because oil-based paint dissolves Styrofoam). Place the balls in the cage and secure the door of the cage with short pieces of wire or twist ties.
(15 minutes or more)
Hold the blower under the cage and blow air up through it. The moving air will agitate the balls, simulating the kinetic behavior of a gas. Watching the colored ball will allow you to follow the motion of a single "molecule."
By adjusting the speed of the blower from the cage, you can simulate heating and cooling a gas. The faster the balls are moving, the hotter the gas.
Listen for the clicking of the balls against the walls of the cage. At lower "temperatures," the clicking is quieter and occurs at a slower rate.
Adding heat (simulated by the blower) to a gas increases its internal energy. The molecules of the gas move faster and strike the walls of their container more often, yielding an increase in pressure (force per area). This increased pressure is simulated by the faster motion of the balls, which strike the sides of the cage more often. Cooling the gas (moving the blower farther from the cage) lowers the internal energy, slowing the motion of the molecules, and thus lowering the pressure.
You may want to try this Snack using cages of different volumes or try nesting baskets to change their volume. In this way, you can model the ideal gas law by changing temperature, pressure, and volume.
If you blow air on one side of the bottom of the cage and not the other, the balls will eventually "condense out." That is, they will form a pile on the side away from the blower, where it is "cooler."