An estimated 12,000 to 15,000 people all exhale marijuana smoke as the clock hit 4:20pm during the 4/20 event at the University of Colorado in Boulder

An estimated 12,000 to 15,000 people all exhale marijuana smoke as the clock hit 4:20pm during the 4/20 event on Norlin Quad at the University of Colorado in Boulder, Colorado April 20, 2010. REUTERS/Mark Leffingwell (UNITED STATES – Tags: POLITICS SOCIETY)