Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. Water vapor accounts for roughly 0.25% of the atmosphere by mass.
Physical property, such as real estate, does not qualify due to the unpredictable length of the sales process. As retirement accounts, 401(k)s and IRAs operate both as liquid assets and non-liquid assets depending on the circumstance.
Money in our bank account is an asset. It's a very liquid asset because we can spend it right now. A building we own is an asset too because it has value - but it's not very liquid. We'd have to sell it (or borrow money against it) in order to use the value to spend it. But we still own it - so it's an asset. Money other people owe us is an asset. The more likely they are to pay us, the better the asset. It's not liquid either because we can't spend money we didn't get yet. Some assets decline in value over time - like your car. The more you drive it, the less it's worth.