The 17th-century English physician William Harvey was the first person to propose that the heart is a pump, acting so as to circulate blood throughout the body. In defense of his radical idea, he offered various bits of evidence, as well as tests his readers could try for themselves. Among the latter, he suggested opening up the chest of a live animal, cutting a small piece from the bottom of its heart, and then sticking one's finger into the heart through the hole one had made. Then the reader would discover that the creature's heart exerted pressure on his finger from all sides, demonstrating its pump-like action.
Why not duplicate Harvey's experiment at home with your own kids, and maybe several of their school chums, so as to give them a first-hand taste of science at work?