Knuckle is the joint where your finger connects to your hand, or where one finger bone connects to the next. When you make a fist, your knuckles are those bumps that stick out on the back of your hand. If you've ever knocked on a door, you were using your knuckles to create that sharp rapping sound.

Your knuckles let your fingers bend and grip, making it possible to hold a pencil, catch a ball, or play piano.

People sometimes say knuckle down when they mean to get serious and focus on work. If you've been putting off your homework and suddenly decide to knuckle down, you're committing to concentrate and get it done. The phrase knuckle under means something different: to give in or submit to pressure.

In games and challenges, kids sometimes play a knuckle-based game where they rap each other's knuckles, which can hurt and cause bruises. And if someone gives you a knuckle sandwich, they're not offering lunch but jokingly talking about a punch.