The human body is designed with joints that allow it to bend, twist and move easily. The hinge joint, for example, enables the elbows and knees to bend back in one direction but not in the other direction. This stabilises the body, allows the potential strength of the arms to be greater and maintains the body in an upright position. You can demonstrate how a hinge joint moves by making a model of a knee joint.
Screw a hooked screw into a tennis ball.
Screw a second hooked screw into the opposite side of the tennis ball.
Hook a bungee cord onto each hook screw.
Slip a PVC pipe onto each bungee cord. This will position the tennis ball in between the PVC pipes.
Stick your finger into the open end of each PVC pipe and grasp the hook of each bungee cord. Pull each cord out and hook to the edge of the respective PVC pipes.
Move the knee joint model to demonstrate how the knee moves inside a human body.