A man exhales after smoking marijuana in a pipe at a pro-marijuana rally at 4:20pm at the University of Colorado in Boulder, Colorado April 20, 2012. The University of Colorado clamped down on a huge annual marijuana fest on Friday by restricting access to the school and a field where the "smoke-in" is traditionally held, and three protesters were arrested ahead of the event. The event was held on April 20 at 4:20pm, a date and time corresponding with a numerical 4/20 code widely known within the cannabis subculture as a symbol for all things marijuana. REUTERS/Rick Wilking (UNITED STATES - Tags: CRIME LAW EDUCATION CIVIL UNREST POLITICS DRUGS SOCIETY)