There are lots of things we can do with balloons. One trick is to blow up a balloon (not all the way) and put it near a source of heat. Remember, if we put it right onto the heat radiator, it will explode. Just put it close to a heat source. After 10 minutes, we'll measure its circumference, and write down the measurement.

Now, we'll get another balloon, and inflate it until it is the same size as the first balloon.

Which balloon has the most air in it? Well, it looks like they have equal amounts of air, but we want to be certain. How can we compare the amounts of air they both have?

Put them on a balance scale and see if one is heavier than the other.

Next, we'll let the warm balloon cool down, and compare the sizes of the two balloons.

You will realize which balloon contained the most air and why one was heavier, even though both were the same in circumference (when the first balloon was heated).

How can it be that the cool balloon weighed more than the heated one even though they were the same size? How can it be that when the heated balloon cooled, it was smaller in
circumference than the second balloon?

My friend, the brilliant scientist, says that identical gases of the same volume may differ in weight due to differences in temperature. This means that the molecules of gases expand when heated.