What is Internal Energy?

Internal Energy is defined as the energy associated with the random, disordered motion of molecules. It refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic. But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second.
                    

So now we can compare the internal energy of 1 gram of water and 1 gram of copper at the same temperature. Earlier we saw that the specific heats of water and metals are different, this is part of internal energy. If the temperatures are the same, the average kinetic energy will be the same, this is also part of internal energy.