Describe a crime to the class. Then two students are nominated (they become the suspected criminals) and they go out of the room and prepare their story of what they were doing at the time of the crime (the stories must be THE SAME), where they went, what they did, what they were wearing, what they ate etc.
The other students are the police detectives and they must write and ask at least two questions to each of the suspects, both about themselves and their partner.
The suspects enter the room one at a time and they are not allowed to speak to each other in between their interrogations. If after both the police interviews the stories are 100% identical (which is rare) they are deemed innocent. If the stories are different, they are guilty!