Note: This episode originally aired in September, 2014.Zoos follow a fundamental principle: You can't sell or buy the animals. It's unethical and illegal to put a price tag on an elephant's head. But money is really useful — it lets you know who wants something and how much they want it. It lets you get rid of things you don't need and acquire things that you do need. It helps allocate assets where they are most valued. In this case, those assets are alive, and they need a safe home in the right climate.So zoos and aquariums are left asking: What do you do in a world where you can't use money?This episode was originally produced by Jess Jiang.Help support Planet Money and get bonus episodes by subscribing to Planet Money+ in Apple Podcasts or at plus.npr.org/planetmoney.