
Let's consider a balloon filled with an ideal gas. When we heat the balloon, the volume increases. When we stop heating the balloon, the temperature has doubled. You will notice that the volume has also doubled.
Let's try cooling as well. We'll return the balloon to its original temperature and volume. Now, let's drop the temperature by half. When the temperature is half of what it was originally, so is the volume.
This has been a simple demonstration of Charles' Law. The movie is in Macromedia Shockwave. There is also a compressed movie in Apple QuickTime format.

Then, place the bottle over the heat source. After a while, the balloon
will expand. Warning: Depending on the heat source, you may need
gloves or guidance. This will look like:

Many people attribute this to "heat rises". However, heat is a property of matter, not a quantity. Therefore, it must be acting on something. The heat raises the temperature of the gas, and according to Charles' Law, expands its volume. The balloon expands because it is the only part of the set-up that will expand.