The Feynman Lectures on Physics

Richard Feynman is the master of all explainers. He takes incredibly complex concepts and break them down to make them more easily digested. Professor Feynman gave a series of seminal lectures at Caltech in the early 1960s that were transcribed and edited, evolving into a book that became the definitive introduction to physics. Now those lectures are available online, for free. Here’s an example:

To illustrate the power of the atomic idea, suppose that we have a drop of water a quarter of an inch on the side. If we look at it very closely we see nothing but water—smooth, continuous water. Even if we magnify it with the best optical microscope available—roughly two thousand times—then the water drop will be roughly forty feet across, about as big as a large room, and if we looked rather closely, we would still see relatively smooth water—but here and there small football-shaped things swimming back and forth. Very interesting. These are paramecia. You may stop at this point and get so curious about the paramecia with their wiggling cilia and twisting bodies that you go no further, except perhaps to magnify the paramecia still more and see inside.

This is an incredible gift to the world.