How does an arm bend?

Bend your arm, then make it straight again. Feel how it folds right at your elbow. You bend it to hug a teddy and to lift a cup.

Your arm feels hard and strong all the way down. So how can it fold in the middle? Let's find out how an arm bends.

Inside your arm are hard bones. One long bone reaches from your shoulder to your elbow. Two more bones reach from your elbow to your wrist. These bones hold your arm up, and they stay stiff and never bend.

All these bones meet at your elbow. Your elbow is a joint, which means a place where bones meet and can move. At this joint, the bones swing like a door on its hinges. That's why your arm folds right there.

Still, a joint cannot move all by itself. Something has to pull the bones. That job belongs to a muscle. This muscle sits inside the top part of your arm, between your shoulder and your elbow.

The bottom end of this muscle holds on to a bone just past your elbow. It holds on tight, like a rope tied to a wagon. That way, when the muscle pulls, the bone has to come along.

When you want to bend your arm, the muscle gets shorter. It scrunches up and gets fatter, like a caterpillar bunching up. When the muscle gets shorter, which way do you think your lower arm will move?

Up comes your lower arm! The muscle pulls that bone up toward your shoulder. Your arm swings at the elbow joint, and it bends.

Let's remember our special word: joint. Your arm muscle gets shorter and pulls your lower arm up at the elbow joint.

Sweet dreams, and tomorrow give someone you love a warm bendy-arm hug.