Watch Your Heartbeat

Bend a paper clip as shown, and slip a soda straw over the upright end. Now locate your pulsebeat with your other hand and tape the paper clip to that place on your wrist in the position shown.

Sit down and rest your arm on a table, holding it as steady as possible. If you have taped the paper clip in the right place, the straw will move back and forth regularly about once every second. Use a clock to count your heartbeat.

Each time your heart pumps blood out into the arteries and blood vessels in your body, the walls of the arteries stretch. Most arteries are down deep inside, but in your wrist there is one so close to the surface that its stretching makes your skin move in and out slightly every time your heart beats. This is called your pulse.

The straw taped onto your wrist is long enough so that a very slight tilting motion of the paper clip causes the top end of the straw to move. This makes it possible for you to see your heartbeat.