How would you explain to a five-year-old about what the heart does for the human body?

The heart basically pumps blood, which has oxygen in it, to power all of your other tissues.

Then once your tissues use up that blood, your blood gets deoxygenated?

It does. And then the heart is still pumping, and that blood basically is in a circuit. It goes through the lungs, and the lungs will exchange the CO₂ out to the air, and you'll get oxygen in it. At the same time, the heart's also pumping that blood to your brain, and then finally to your kidneys and your other organs, the liver to detoxify your blood, the kidneys to basically pull out any metabolic artifacts and then excrete them through the urine. It's a critical piece to the machinery. Without the heart, you won't live.