**Comments:**

To understand the article on gauge theory, you have to know quantum mechanics and what a vector potential is. Careful readers will find that the connection introduced in this article is precisely the electromagnetic potential 1-form explained in “Maxwell’s equations in differential forms.” Similarly, they will find that the notion of gauge transformations in both cases coincide. “Feynman path integral” shows how quantum mechanics can be as bizarre as it can be. Familiarity with Lagrangian, Hamiltonian and quantum mechanics is the prerequisite. Special relativity as taught in a freshman physics class, Einstein summation convention, Lagrangian mechanics are the only prerequisites to “Introduction to general relativity,” even though familiarity with Maxwell’s equations is required to understand section 5, which can be skipped for those who aren’t familiar, and the article “Feynman path integral” is needed to understand section 29 which deals the temperature of black hole. In this article, I tried to write only the essentials rather than being complete, to make it as easy as possible. I hope that this article may serve as a steppingstone to more advanced material. “Dirac string” shows that a possible existence of magnetic monopole necessarily implies the quantization of electric charge.