If they guess correctly, they change places with the sender. Go on until you decide to stop. It is a muscle which is about as big as a person's fist. Inside the body are many tubes called arteries and veins. The heart pumps blood through these tubes.
Blood carries oxygen from the heart to all parts of the body.
Listen to own heartbeat with a stethoscope. Describe the sound. Listen to the heart again. It is beating faster because more oxygen is needed by the body. Compare and contrast human heartbeat to heatbeat of a class pet.