The Prisoner's Dilemma

Imagine you're one of two prisoners. You're both caught for a crime, but the police don't have enough evidence to convict either of you on a serious charge. They put you in separate rooms and offer each of you a deal. If you betray the other and they stay silent, you go free and they serve a full sentence. If you both betray each other, you both get a moderate sentence. What do you do?