Your knee is the joint in your leg that joins your thigh with your shin, making it possible for you to bend your leg. Without knees, you'd walk like a Barbie doll on very stiff legs.
The knee connects your tibia, or shinbone, to your femur, or thighbone. Although their bodies are arranged in different ways, there are other animals with knees, including horses, donkeys, giraffes, and deer. You can also use the word knee to mean the part of your pants that covers your actual knee: "I always get a hole in the knee of my jeans." A "knee-slapper" is a joke so funny it makes you laugh out loud.