Please answer the following question: Generate a question about the following movie plot: In a newspaper advertisement, taxi driver Tarek Fahd discovers an invitation to participate in an experiment. 4000 German marks are offered to the participants of the experiment, in which a prison situation is simulated. The experiment is led by Professor Klaus Thon and his assistant, Dr. Jutta Grimm.Tarek participates as a journalist while wearing a pair of glasses with a built-in mini-camera. Tarek has a car accident shortly before the experiment, after which he meets a woman called Dora. She spends the night with him and Tarek keeps thinking of her, shown in flashbacks. The 20 volunteers are pronounced guards and prisoners and are being observed by a team of scientists. The prisoners lose their civil rights and have to obey arbitrary rules, such as completely eating their meals. The guards are given nightsticks but are told not to use violence in any case. Each prisoners name is taken away and replaced by a number. Tarek (prisoner nr. 77) initially refuses to acknowledge the guards' superiority by drinking the milk of one of his co-prisoners because of that prisoner's lactose intolerance or by throwing his blanket out of his cell to provoke the guards. He befriends his cellmates, Steinhoff and Schutte. Psychological changes develop and the situation deteriorates. The circumstances seem to be escalating after a few days. It becomes clear that limits are not only being reached but being surpassed.The guards become excessively aware of their power and use the prisoners' fear to make them obedient. On both sides, one person is considered dominant. On the prisoners' side, this is Tarek, and aon the guards side, it is the quiet guard Berus, a sadist, whose motto during the experiment is: "Humiliation is the only way we can solve these troubles." From that moment, the guards start to use more and more violence against the prisoners. The scientists engage in a discussion whether or not to abort the experiment. Dr. Grimm suggests to put an end to the alarming situation, but Professor Thon refuses to stop the...
A:
What rule do the prisoners must obey