Air density

Air density is like how tightly packed a bunch of balloons are in a big room. When there are a lot of balloons in the room, they are tightly packed together and it can be hard to move through them. This means the air density is high. When there are only a few balloons in the room, they are spread out and it's easier to move through them. This means the air density is low.

Similarly, air density is how many air molecules are packed into a certain amount of space. When there are a lot of air molecules in a certain area, the air is more dense. This can affect things like how planes fly, because denser air can provide more lift for the plane. It can also affect how well our bodies can breathe, since denser air can be harder to move through.