A human with the metabolism of a hummingbird would need to eat about 80,000 calories a day to survive. That’s because staying warm by making your own heat takes a ton of energy. And that brings up a paradox, because creating your own body heat is incredibly expensive. The largest animals on Earth have to spend most of their day just eating enough to keep their heat engines running. So why aren’t we cold-blooded… like crocodiles? Here’s the science