Five thousand years ago, English, Spanish, Russian, Hindi, and dozens of other languages across Eurasia were all a single language called Proto-Indo-European, which was never written down and which nobody still speaks. How can we claim the existence of a language that was never written down? How do we know that modern languages were once the same language, and how exactly can languages differentiate and change so drastically throughout history? In this introduction to Historical Linguistics, we'll answer all these questions and more and learn the meaning behind specific terms like "sound change," "analogy," and "(linguistic) genetic relationship."