Can't is a contraction of “cannot,” meaning to be unable to do something or not to be allowed to do it.

When you say you can't reach the top shelf, you're physically unable to. When you say you can't go to the party because of your curfew, you mean you're not allowed. The word works for both situations: things that are impossible and things that are prohibited.

People sometimes confuse can't with won't. If you say you can't clean your room, you're claiming it's impossible. But if you actually mean you don't want to, the honest word is won't. The difference matters because can't suggests a real obstacle, while won't means you're choosing not to.

Sometimes can't reflects a temporary limit you can overcome with practice. A beginning piano player can't play Beethoven yet, but with months of work, that can't becomes can. Other times, can't describes permanent limits: you can't be in two places at once, and you can't change what happened yesterday.