Open-and-shut describes a situation, case, or decision that is completely obvious and requires no debate or investigation. When something is open-and-shut, the answer is so clear that anyone can see it immediately.

In courtrooms, lawyers sometimes call a case open-and-shut when the evidence points overwhelmingly in one direction. If security cameras caught someone stealing a bicycle, witnesses saw it happen, and the thief was found riding that exact bike an hour later, that's an open-and-shut case. The jury doesn't need to puzzle over conflicting stories or evaluate complicated evidence.

The term comes from the idea that you can open something, look inside, and immediately shut it again because you've seen everything you need to see. You might use it when your teacher asks who left the art supplies scattered everywhere, and there's paint all over one student's hands. Or when your mom asks who ate the last cookie, and your little brother has chocolate smeared across his face. Those are open-and-shut situations.

Be careful, though: what seems open-and-shut to one person might not be so simple to someone else. Sometimes people rush to judgment and call something open-and-shut when they should really look more carefully at the facts.