Recapping the content of 2.1: Ancient Greeks guessed that all matter consists of tiny, identical, unbroken atoms; Dalton confirmed this idea with quantitative experiments and proposed that different elements have different atoms; Mendeleev discovered that all elements are related to each other; Thompson proved the existence of electrons; Rutherford established the existence of nucleus; A planet model of the atom was proposed that can explain the spectrum but cannot explain the stability of atoms—why a supposedly accelerating electron does not radiate energy and crashed into the nucleus. This puzzle led to the birth of quantum mechanics. We concluded 2.1 by mentioning that the order of making measurements is important, or roughly speaking, the order of doing things is important. Making love before dating and dating before making love may have different consequences as we all know. That’s what quantum mechanics all about, if you think hard enough. Therefore, quantum mechanics is not that outlandish. People talking about it but not understanding it make it sound so. Now let me show you why quantum mechanics is simple and it should be the way our nature operates.