Femur

The femur is the thighbone in tetrapods. It connects to the pelvis (hip) at the upper end and to the knee at the lower end. It connects with the hip by a ball-and-socket joint which allows freedom of movement. 
The lower end of the femur forms a hinge with the lower leg known as the knee. This has two 'articulations': with the patella (knee-cap), and with the tibia (the main lower leg bone).The femur is also the longest and strongest bone in your body.