The concept that air exists but we can't see it is hard for children to learn or understand.
Introduce the concept of air with fun science experiments and activities. Watching and performing these experiments your child will develop an understanding of the properties of air, that air has weight, it occupies space, it can move and it exerts pressure.
Learning about air and its properties is the first lesson in physics, which will lead to learning about other gases like oxygen, carbon-di-oxide etc. and understand topics like gravity. It can also lead to learning about environment-friendly topics.