What's the most mind-blowing fact you've ever learned that made you question everything you thought you knew?

One of the most mind-blowing scientific discoveries that made me question my existing knowledge was the concept of relativity, formulated by Albert Einstein. Learning about his theory of special relativity, where space and time are interconnected and can vary depending on an observer's motion, completely challenged my preconceived notions about the nature of reality. It made me question the idea of absolute space and time that I had previously taken for granted. This revelation highlighted the profound impact that our perspective and frame of reference have on how we perceive the physical world.