NPC: Air.

Air is a mixture of many gases and tiny dust particles. It is the clear gas in which living things live and breathe. It has an indefinite shape and volume. It has mass and weight, because it is matter. The weight of air creates atmospheric pressure. Atmosphere is a mixture of about 78% nitrogen, 21% of oxygen, and 1% other gases, such as Carbon Dioxide. Air is invisible: it cannot be seen by the eye, though a shimmering in hot air can be seen.[1] Air is one of the 4 classical elements.