Young people are frequently the target of police harassment and abuses of power. The assumption seems to be that young people are generally up to no good, and that they should be considered guilty until proven innocent. Curfews in some areas target minority youths for discriminatory treatment. Young people find themselves receiving citations for lawful activities such as talking to their friends on a street corner. A common tactic is to stop a car full of young people on the basis of some minor infraction-- something hanging from the rearview mirror (yes, that's illegal!), for instance, or a non-working tail light. From there, the officer might say something to the effect of "Do you mind if we have a look around your car?" Making it clear from the tone that you really don't have a choice. The trick is, that while it's true that you may not be able to stop them from searching the car,you have the right to refuse to give your consent to a search. If you state that you are not consenting to a search, and they proceed to search anyway, the police must establish that they had probable cause to believe you committed a crime, or the search is illegal.
