"If you can explain this using common sense and logic, do let me know, because there is a Nobel Prize for you.." Professor Jim Al-Khalili explains the experiment that reveals the "central mystery of quantum mechanics" - the double slit experiment. Sometimes called the "two-slit" or "Young's" experiment, it demonstrates that matter and energy can display the characteristics of both waves and particles, establishing the principle known as wave-particle duality. Furthermore, it questions the role of the observer in the outcome of events and demonstrates the fundamental limitation of an observer to predict experimental results. For this reason, Richard Feynman called it "a phenomenon which is impossible ... to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics]."