An impossibility is something that cannot happen or cannot be done, no matter how hard you try or how much you want it. It's impossible to travel backward in time, impossible to count to infinity, and impossible to be in two places at exactly the same moment.

Some impossibilities are physical: you can't make water flow uphill without a pump, and you can't make two plus two equal five. Other impossibilities exist because of the rules we've agreed on: it's impossible to win a chess game by moving your pieces however you want.

Throughout history, people have sometimes called things impossibilities that turned out to be possible after all. Flying seemed like an impossibility until the Wright brothers built an airplane. Running a mile in under four minutes was considered an impossibility until Roger Bannister did it in 1954. These weren't true impossibilities, just things that were extremely difficult or that no one had figured out yet.

When someone says “that's an impossibility,” they mean it genuinely cannot be done. But when they say something “seems like an impossibility,” they might just mean it's extremely hard. The difference matters: true impossibilities help us understand the limits of reality, while seeming impossibilities often show us where we haven't tried hard enough yet.