Professor Susskind begins the lecture with 2 examples: (1) deriving the speed of sound in an ideal gas; and (2) a single harmonic oscillator in a heat bath. The harmonic oscillator example leads to a discrepancy with empirical observation that can only be resolved through quantum mechanics. At low temperatures relative to the first excited state of the oscillator, quantum mechanics suppresses the energy of the harmonic oscillator. Through this mechanism, certain modes of oscillation are "frozen out" until the system reaches higher temperatures. Einstein proposed this quantized effect in 1907, which is one of the theories that led to the development of quantum mechanics.