New York City made another attempt to improve the health of its citizens yesterday, as a ban on trans fats in the city's restaurants, cafeterias, and even hot dog and pretzel stands took full effect. Trans fat, also known as partially hydrogenated vegetable oil, is linked to clogged arteries and heart disease. But restaurants have found many uses for the man-made vegetable shortenings and oils, which give pastry its flakiness and french fries their crispiness. It's the first such ban by a major U.S. city, and most give credit for the idea to New York Mayor Michael Bloomberg, who has been on a health crusade during his tenure in office.