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.
So it's probably a good idea to take stock of what you have right now in liquid assets. But what counts as a liquid asset really? Perhaps appropriately, the definition of a liquid asset is not fixed in stone, but can vary.