The air is for us a matter of course. He's everywhere and does exist, even though we do not see. If he moves, we feel it as the wind.
But to imagine something invisible is difficult. How can you prove the presence of air? With some easy experience: in the bowl of water to dip the bottom up the glass and press it, then we will see that the water in it does not penetrate.
Why? "Empty" glass is not actually empty. It is filled with air.
Water may enter only then, when we get to immerse it in an inclined position, allowing the water to displace air.