Heat will flow from a hot substance to a colder substance unaided, but it is impossible for heat to flow from a cold substance to a hotter one without the aid of mechanical work is:

The statement describes a fundamental principle about the direction of heat transfer, which is governed by the Second Law of Thermodynamics. This law states that heat will naturally flow from a region of higher temperature to a region of lower temperature, and it emphasizes that heat cannot spontaneously flow in the opposite direction— from a colder body to a hotter body— without some form of external work being applied.

This phenomenon is why refrigerators and heat pumps require energy input: they are designed to move heat away from a cooler interior space to a warmer environment, effectively reversing the natural heat flow, but only by expending energy to do so. Thus, the understanding that spontaneous heat transfer occurs only from hot to cold and a mechanical process is required to reverse that flow is a core aspect of the Second Law of Thermodynamics.

The First Law of Thermodynamics focuses on the conservation of energy, saying that energy cannot be created or destroyed, only transformed from one form to another. Charles' Law relates specifically to gas behavior under changing temperature and pressure, while the concept of the mechanical equivalent of heat refers to the relationship between mechanical work and thermal energy, which does not directly address the unidirectional nature of heat flow as stipulated in the Second Law.