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What Is Déjà Vu?

If you’ve ever been going about your day as usual, only to be struck by the sudden feeling that you’ve somehow—however improbably—already experienced what you’re going through, then congratulations: You’ve just experienced déjà vu. You might even know the name for the feeling. But what is déjà vu, really? And what could possibly explain this weird and unsettling experience?

4 of History’s Strangest Scientific Theories

Throughout the entirety of human history, we’ve strived continuously to understand the world—to come up with plausible explanations for natural phenomena that we may not fully grasp. Alas, as some of history’s strangest scientific theories demonstrate, we don’t always get it right. In fact, we’re frequently so wrong that many of these now-obsolete theories edge into outright absurdity.