This lecture introduces the Einstein model, which describes the vibrational motion of atoms in a solid as quantum harmonic oscillators with the same frequency. It explains how to calculate the specific heat of a solid using this model, defining the Einstein temperature and deriving the energy levels. The lecture covers the calculation of the vibrational frequency and energy gap for aluminum, the internal energy of a solid, and the graphical representation of internal energy as a function of temperature. It also discusses the behavior of specific heat at high and low temperatures, in accordance with the Dulong-Petit law. The lecture concludes with a qualitative representation of the solid's specific heat as a function of temperature.