The patella is the protective bone at the front of your knee. Next time you bang your leg on the edge of the coffee table, you can say, "Ow, my patella!"
The patella is more commonly called a "kneecap." This bone resembles a triangle with rounded corners, perfectly shaped to protect your knee joint from injury. When you were born, your patella was just soft cartilage, but it hardened as you grew. Like humans, many other animals also have patellas, including mice, dogs — and even birds!