Ask your victim, er.. friend to put a quarter on a piece of paper and, without removing their finger, trace the coin with a pencil or marker. Repeat the "test" with a few fingers. After that's done, get the victim to pick up the quarter and roll it along the bridge of their nose. Then quietly snicker behind their back as they walk around with a black line along their nose. Don't use a permanent marker beacuse that's not cool.