If you’re in a car, you see watches outside the car go slower. Lots of people thought it must be wrong because it’s so strange, but it turned out to be right. The special idea is really, really strange, and understanding it can take a lot of work. To understand the big idea-the hard one-it helps to understand the special idea first. People who are good at numbers can use the special idea to answer questions pretty easily, but you have to know a lot about numbers to do anything with the big idea. The big idea is a lot harder to understand than the special one. The other one-the big idea-covers all the stuff that is left out by the special idea. The first idea is called the special idea, because it covers only a few special parts of space and time. People have spent a lot of time trying to figure out how he was so good at thinking.) (He also had a few other ideas that were just as important. That second idea was a hundred years ago this year.

This thing you’re reading right now explains those ideas using only the ten hundred words people use the most often.1 The doctor figured out the first idea while he was working in an office, and he figured out the second one ten years later, while he was working at a school. Two of his biggest ideas were about how space and time work.

He was so good at coming up with ideas that we use his name to mean “someone who’s good at thinking.” He didn’t grow the cool hair until after he was done figuring that stuff out, but by the time everyone realized how good his ideas were, he had grown the hair, so that’s how everyone pictures him. He was well known because he came up with some important ideas. There once was a doctor with cool white hair.

Xkcd munroe