All elements can exist as either a solid, liquid, or gas, but very few exist in all of these forms naturally. Oxygen is no different. While water, which is made up of mostly oxygen, is one of the rare substances that exist in nature in every form, oxygen itself exists only as a gas. However, the technology exists to bring oxygen to low enough temperatures and high enough pressures to condense into its liquid form.
Liquid oxygen displays a number of interesting properties. Because it has to be so cold, it will freeze anything it comes in contact with, and can cause it to become extremely brittle. It is also a powerful oxidizing agent, causing organic material to burn rapidly and energetically when put together. Some materials become very unstable and unpredictable when soaked in liquid oxygen, detonating in some cases just from light impact.
Liquid oxygen is classified as an industrial gas and is used for a number of industrial and medical purposes. It is a common liquid propellant in spacecraft rockets, usually combined with liquid hydrogen or kerosene. Many early rockets used liquid oxygen, and modern rockets, including the main space shuttle engines, still use it today. It has also been used in explosives, although it is rarely used now due to its instability.
Liquid oxygen can be used as a source of breathing oxygen, and many commercial and military aircraft utilize this ability. When it is in liquid form, oxygen is 861 times more compact than the gaseous form. Because of this, large quantities can be used in a relatively small space. Oxygen has a light blue color, which accounts for the color of the sky.