The arm, with its strength and midpoint joint, is one of the more fascinating parts of the human body. To get a better idea of how its muscles work together to give it strength and flexibility, you can build a model using some household items.
Glue the ball to the end of a toilet paper tube. Most of the ball should be on the outside of the tube.
Poke a hole in the tube with the pushpin, about 3 centimetres from the open end of the tube. Poke another hole in the same spot on the opposite side.
Poke holes on opposite sides of the other tube, about 5 centimetres from one end.
Unbend the paper clips, and slide one into each tube through the holes made by the pushpin, leaving equal amounts of the paper clips on both sides of the tube. Fold those sides of the paper clip up toward the closer end of the tube.
Select one of the toilet paper tubes to work with. Hang a rubber band from each protruding end of the paper clip. Then use the masking tape to secure the two rubber bands to the paper tube.
Place the two tubes together with the ball in the middle. The paper clipped ends should be facing away from the ball. Make sure that one end of each paper clip is facing up while the other end of the paper clip is facing down.
Stretch the rubber bands that are taped to the paper tube down the model, and hook them to the corresponding paper clips on the other tube. Tape the rubber bands in place on the second tube with masking tape.
Tape the top rubber band in place 5 centimetres below the ball on the glued paper tube. Separate the sides of the rubber band so that it wraps around the ball, then tape it to the ball. The lower part of the "arm" should have the tube with the ball glued into it.
This works best with two people, one following the steps, the other holding the model.