Over the course of November 1915, the German physicist Albert Einstein gave a series of lectures in which he presented his theory of general relativity to the world for the first time.
Einstein's ideas explain one of the biggest questions in science: how gravity works. His theory is now the underpinning of much of modern physics.
It is also famously hard to understand. So BBC Earth's Pierangelo Pirak and Melissa Hogenboom sat down with science writer and astrophysicist Adam Becker. Over the course of five minutes Adam explained the whole thing, using nothing more complicated than a laser pointer, an elevator and a few clocks.
Watch the video above to discover how Einstein's big idea really works.