"The material I'm working with is people -- creating moments for them to be thoughtful," says Britta Riley, whose window farms have been displayed in more than a dozen buildings in New York City. Riley and her collaborator, Rebecca Bray, are conceptual artists whose goal is to engage the public in developing simple solutions to vexing environmental problems. By artfully demonstrating how lettuce and tomatoes can be grown in even the most cramped urban spaces, they hope to inspire people to think about where their food comes from -- and then take part in producing it. Riley and Bray constructed an installation similar to the one shown here for the Whitney Museum of American Art and offer how-to tips at windowfarms.org.