Heating a body does result in a slight increase in mass due to mass-energy equivalence, as expressed by E=mc^2. This effect is minimal because the speed of light (c) is a large number, making the mass change negligible. The concept of mass is complex, as it is not solely dependent on the quantity of matter but also on energy and momentum, particularly in relativistic physics. The invariant mass provides a more consistent definition across different frames of reference, distinguishing it from the concept of relativistic mass. Understanding these principles requires a grasp of both classical and advanced physics concepts.