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Cold refers to the condition or subjective perception of having low temperature, the opposite of hot.[note 1]
A lower bound to temperature is the absolute zero, defined as 0 K on the Kelvin scale, an absolute thermodynamic temperature scale. This corresponds to "273.15 °C on the Celsius scale, "459.67 °F on the Fahrenheit scale, and 0 °R on the Rankine scale.
Since temperature relates to the thermal energy held by an object or a sample of matter, which is the kinetic energy of the random motion of the particle constituents of matter, an object will have less thermal energy when it is colder and more when it is hotter. If it were possible to cool a system to absolute zero, all motion of the particles in a sample of matter would cease and they would be at complete rest in this classical sense. The object would be described as having zero thermal energy. Microscopically in the description of quantum mechanics, however, matter still has zero-point energy even at absolute zero, because of the uncertainty principle.
Mammals have an endothermic ("warm-blooded") physiology. Therefore they have a perception of thermal comfort that is centered around their body temperature. Their thermal comfort is dependent upon their metabolism and environmental surroundings, and is affected by heat conduction, convection, radiation, and evaporative heat loss. Thermal comfort is maintained when the heat generated by their metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. When this equilibrium is not maintained due to excessive heat loss, then they will perceive the feeling of being too cold, and in varying degrees of discomfort depending upon how far this equilibrium is skewed.[citation needed]
Cooling refers to the process of becoming cold, or lowering in temperature. This could be accomplished by removing heat from a system, or exposing the system to an environment with a lower temperature.
Fluids used to cool objects are commonly called coolants.
Air cooling is the process of cooling an object by exposing it to air. This will only work if the air is at a lower temperature than the object, and the process can be enhanced by increasing the surface area or decreasing the mass of the object.
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