Instantly? Just like if you go outdoors when the temperature is below zero (centigrade) you freeze instantly? Are you from a location where temperatures never drop below freezing and simultaneously unaware that people live in places where the temperature is below freezing? Otherwise, I can't understand your confusion. Given perfect insulation (which doesn't exist) a material can theoretically maintain its temperature indefinitely. With real world insulation colder materials will slowly warm up, but the rate its temperature increase depends on the quality of insulation. A thermos of a hot liquid can still burn skin many hours after it has been filled. All substance have some ability to insulate, so a liquid or gas can insulate itself (that is can insulate material further away from the heat source). In beginning physical science, you should have learned that a material's temperature is an indication of its energy content. You should also have learned that energy can be neither created nor destroyed; which means that for a material to warm up, energy to do that has to come from somewhere. In terms of heat flow, there are three types of flow: Conduction, convection and radiation. A cup of lox (liquid oxygen) on a table, say, will mostly be warmed by conduction. (Unless you place it under intense lighting or near some other energy source.) As the lox heats up, the table cools down, meaning it will take longer and longer for heat to move into the lox from further away-heat conductivity is slow compared to the speed of light, speed of sound, or even a speeding bullet. (Depending on the shape of the cup, the air above the lox might also supply heat via turbulent flow) One good example of heat flow differences is when you walk barefoot over carpet, it feels warmer that walking over ceramic, wood, or metal even though they are at the same temperature! (or if the surface is hotter than your skin, the carpet will feel cooler than the other surfaces) The heat conductivity (not really related to electrical conductivity!) is faster in metal, and slower in carpet (metal>ceramic>wood) (plastics vary because their composition can be quite different, but organics are generally slower than metals and ceramics (including rock)). As far as what happens to a solid under pressure when the pressure is relieved, there is no single answer. It depends on the strength of the material. If the material is weaker than the forces of expansion, then it will puff up a little and this will probably turn a solid piece into a pile of powder. If the material is strong enough, it won't change its shape much until it begins to warm up. I don't know what the tensile strength is of solid oxygen. Note that compression heats a material and expansion COOLS a material (in general), so releasing the external pressure on an object will tend to cool it. As a material warms, its dimensions change. Perhaps you've seen the failure of a glass container when it is subject to too much temperature change too quickly? It cracks. Stress may cause the material to, drum roll...crack. Finally, for a gas (or liquid) which is above the temperature and pressure at which it exists (at equilibrium) as a gas (or liquid) but has been compressed to a solid, then releasing the pressure will allow mechanical stresses to be relieved and at the same time will cool the solid. The forces involved in containing a material as a solid when it is at a temperature at which is would normally be a gas are quite high (usually). How fast these forces are released depends not only on the material but on the amount of material. Usually the compression required means the sample is really small, fractions of a gram. With such small samples, the phase change will be very rapid (since conduction depends on the surface area, and small samples have high surface area per unit mass. LNG (liquified natural gas) is a counter example. In many parts of the world, tanks of LNG can be seen almost anywhere. A (small) opening will allow the liquid to slowly gassify. If a large opening were provided, I don't know how fast a tank's content would evaporate; the evaporation front would be exerting a compressive force on the underlying liquid, and along with the cooling due to expansion, the material would PROBABLY not evaporate "instantly". Given that a material moving by a surface will tend to create static electricity, and given the explosive nature of alkane/air mixtures, that's NOT an experiment I'd want to try. Finally, note that a material which doesn't decompose as it is heated eventually reaches a temperature, called its critical temperature, above which the liquid state doesn't exist - a solid above this temp will sublime as it changes phase. Throwing lox into the air vastly increases its surface area, and allows the small droplets to rapidly warm (and turn into gas).