The knee is the joint in the middle of your leg, connecting your thigh bone to your shin bone. It's what lets you bend your leg to sit, run, jump, and kick. Without knees, you'd have to walk around on stiff, unbendable legs like a toy soldier.

Your knee is one of the largest and most complex joints in your body. It works like a hinge that can swing back and forth, though it also twists slightly when you turn. Inside, there's a small piece of cartilage called the meniscus that acts like a cushion between the bones, plus ligaments that hold everything together. Athletes often injure their knees because they put so much stress on these parts during sports.

When someone asks you to do something difficult or unpleasant, they might say they're asking on bended knee, an old phrase meaning to beg humbly. And when something happens automatically without thinking, people say it's a knee-jerk reaction, named after what happens when a doctor taps just below your kneecap and your leg kicks forward on its own.