If you cut the holes a bit too big, use tape to patch the side of the straws. Also, tape the end of one of the straws.
Now it's time to pump your heart! Place your heart pump on a tray (this is going to get a little messy!). Use your fingers to press down into the balloon between the two straws. Watch the blood pump out from the pink straw!
Of course, the action of a real heart is much more complicated than this simple model, but your heart really is a pump! With every heartbeat, muscles in your heart contract and squeeze the chambers (or open spaces) within your heart, just like your finger poking into the balloon. This squeeze pushes the blood in the chamber and moves it out of your heart and into your arteries. In your balloon heart, the water "blood" gets pushed up and out the straw "artery". Your heart does this an average of 70 times every minute, pushing your blood all the way around your body and back to your heart again!