Mischief is playful behavior that causes minor trouble or annoyance, but without any real harm or mean spirit behind it. When your little brother hides your backpack before school, that's mischief. When students switch the classroom clock forward by five minutes hoping to end class early, they're making mischief.

The word suggests a kind of harmless fun mixed with rule-breaking. Someone who's mischievous enjoys stirring things up or testing boundaries, but they're not trying to hurt anyone or cause serious problems. A mischievous smile usually means someone is planning something playful. The twins in many children's stories get into all sorts of mischief: sneaking cookies, playing pranks, or creating small chaos that adults have to sort out.

There's an important difference between mischief and truly bad behavior. Hiding someone's pencil case might be mischief, but stealing it would be theft. Putting a whoopee cushion on a chair is mischievous, but breaking someone's property is destructive. The line matters: mischief makes people laugh or roll their eyes, while real wrongdoing causes genuine harm.

When adults say someone is “up to mischief” or “getting into mischief,” they often say it with a little smile, because mischief is part of being young and learning how the world works.