Here are two images of a right leg that is bent at the knee. The leg on the left doesn't look very bent, in fact it almost looks like a deformed leg that is straight. On the right is the same drawing, but the shadows on the shin have been darkened and the leg looks bent at the knee. The shadows on the thigh are the same in both images, only the shin shading has been changed. In the right image, the foot also looks like it is pointing more towards your eye. This is because the contrast on the leg has been heightened, but the lighting on the foot and thigh are pretty similar, so your brain says they are at similar angles and the shin is at a much more extreme angle than the foot and thigh.