Heat is Kinetic Energy

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What is heat really? Is it an invisible fluid that your bare feet soak up from a hot sidewalk? Is there a maximum to the temperature scale? The theory of heat as a fluid seemed to explain why colder objects absorbed heat from hotter ones,but once it became clear that heat was a form of energy, it began to seem unlikely that a material substance could transform itself into and out of all those other forms of energy like motion or light.