And so, getting back to NASA, if we want to know whether the space agency will complete the ISS, we should consolidate into a single price all the available knowledge about the project and then start trading. Here's how it works: We create a security (the "proposition") that asserts: "NASA will complete building the ISS by the end of 2010." If, at the end of 2010, NASA has indeed completed ISS construction, everyone holding that security receives $100. But if it does not finish, everyone holding that security gets zilch. Between now and then, anyone can buy and sell that proposition for an ever-fluctuating amount of money between $0 and $100. If you think it's 60 percent likely that NASA will finish, you'd be willing to bet up to $60 in order to earn that $100. Therefore, you'd buy that proposition if it was selling for $60 or less. If you lost confidence, you'd look to sell.