How does the internal energy of a body change if it is: a) heated; b) cool?

a) increases; b) decreases.
In a solid, particles interact: they attract and repel. At the same time, they perform oscillatory movements. With increasing temperature, the average kinetic energy of the vibrational motion of its particles increases. Since the particles interact, the vibrations of neighboring particles are amplified, and from them – the next, etc. all over the bar. With this type of heat transfer, the transfer of matter does not occur. In gases, energy is transferred only due to the chaotic movement of molecules and collisions between them.