When one asks a person: "What is air?", many times this answer is heard: "Nothing, or virtually nothing". Explain to the audience that air consists of small particles, molecules. Let the audience estimate how much the weight (mass) is of 1 m3 of air.
A glass bottle, that contains 1 liter of air, is placed on a balance. The weight is read from the display or scale.
The bottle is now evacuated and put on the balance again. The weight is read again.
Observe how the weight is diminished. The difference in weight, read from the scales, is about 1,3 g. When the air is admitted to the bottle again, the original weight reappears.
Explanation 1, 2 :
At the first measurement the total weight of the bottle and air together is determined.
Then, by means of a suction, the air is taken out of the bottle. When the bottle is weighed again, only the weight of the bottle itself is measured. The difference in weight is the weight of the air that was in the bottle. In this measurement the result is: the weight (mass) of 1 liter of air is about 1,3 g. As 1 m3 contains 1000 liter, the amount of air in a volume of 1 m3 is 1000 times as much as in 1 liter. So 1 m3 of air weighs about 1300 g, that is about 1,3 kg, which is rather substantial!
(In this explanation the words 'weight' and 'mass' are carelessly intermingled).