You may have heard the phrase "cap and trade" recently. It's one way we could rein in the greenhouse gases that are throwing our climate off-balance. What we'd do is this: set an economy-wide limit on emissions (the "cap"), and then let companies and industries buy and sell a limited number of permits to pollute (the "trading"). The free market would determine pollution's cost. But for industries that have been polluting for free since industry began, suddenly having to pay for it doesn't seem like such a great deal. Every industry will argue that it should get a chunk of the pollution permits for free. Figuring out how to allocate emissions permits isn't going to be easy, but it'll be one of the most important steps in actually getting a cap and trade system in place. If you've watched all the day's viral videos and want to get a little more detail on how we can save the planet, this manageably wonky piece from WorldChanging will leave you feeling like an expert on pollution permit allocation.