Penelope Cruz, Will Ferrell, Jessica Biel and Tobey Maguire are among the Hollywood stars who have filmed environmentally friendly public service announcements, which will air during next weekend's (07Jul07) Live Earth marathon. The movie stars will trade green tips during the PSAs, in which they each contribute something they do on a daily basis to help the environment. In the short black-and-white spots, Cruz makes sure her air conditioning is never on for too long, British actress Emily Blunt heats only enough hot water for her cup of tea, while Ben Affleck promotes downloading music as a way to reduce costs associated with manufacturing and shipping CDs.