Heat can be transferred from a hot object to a cold object by bringing them into contact with one another. This happens because the atoms of the hot object physically collide with the atoms of the cold object, and thereby transfer kinetic energy. While a transfer of thermal energy is one way to change the heat content of a sample, it is not the only way. For example, the sun produces light that is converted to heat on earth. However, the sunlight is a form of potential thermal energy because it can be converted to thermal energy.
Chemical bonds represent another form of potential thermal energy.
When bonds are formed or broken, there is usually an energy change, and that energy is often converted to heat. This brings us to a useful working definition of enthalpy. The symbol H is almost always used to represent enthalpy. Unfortunately, it is not possible to directly measure the total enthalpy of a system. The reason for this is that we cannot accurately measure the potential thermal energy of a system because we cannot know what portion of its total energy will be converted to heat, and what portion to other forms of energy.