Answer:
They have a very low density
They fill the container they are in
Explanation:
The first main characteristic is that they have a very low density, which is logical considering that gasses are lighter than the other states of matter (solid and liquid), however some gasses are extremely light, and some very heavy (by the relative measurement of other gasses). For example Helium is lighter than Nitrogen, so therefore we will breathe in more nitrogen than helium.
The majority of gasses are restricted to the "container" of our atmosphere, which explains why gasses never stay still, they are all trying to spread out evenly around the planet.
