The problem starts when being polite and forgettable puts you on their radar. Police look in your backpack, and hey - it's a bike lock just like the one that was used to assault someone down the block a few minutes ago! You look about the right height. Which direction did you say you were coming from again?

Some situations can be lose-lose. Best you can do is don't talk to the police, and if you absolutely must, say as little as possible and don't lie.