How do you explain Einstein’s Theory of Special Relativity when you don’t allow yourself to use the word “relativity”? Or “gravity”? Or “Einstein”? Or pretty much everything else you find in a science textbook?
That’s the challenge Randall Munroe (creator of XKCD) gave himself in an essay for the New Yorker, where he limited his explanation to the one thousand most commonly used words in the English language: