And speaking of pollution bleeding over borders, Grist's InterActivist this week is an expert on the topic. D. Rick Van Schoik is managing director of the Southwest Center for Environmental Research and Policy, which works on pollution and environmental problems that straddle the U.S.-Mexico border. Today he answers our questions, touching on everything from population and gas-guzzlers to cigars, cars, and "The Simpsons." Check out his answers, and shoot him a question of your own -- only on the Grist Magazine website.