The longest bone in the human body is the femur, commonly known as the thigh bone. The femur plays a critical role in supporting the body's weight during standing, walking, and running. It connects the hip joint to the knee joint and is designed to withstand significant stress due to movement and weight-bearing activities. Typically, the femur measures about 18 inches (46 cm) in length in adults, but this can vary based on an individual's height. Its unique structure allows for a wide range of motion while providing stability, making it crucial for mobility and physical activity.