Think of an upper-middle class high-school boy who just got his driver's license. One day after lacrosse practice John decides to go to his friend's house instead of straight home. While there, his friend offers him some weed. He hasn't smoked before, but he's heard it is relatively safe and won't hurt him, so he smokes a joint with his buddy. On the way home, a police officer pulls John over for erratic driving. The cop sees his bloodshot eyes and smells the marijuana on his clothes, and he arrests John for driving under the influence.
Source: Washington Free Beacon