This is a fun activity for the students to practice using the preterito and imperfecto in a real time event that takes place in the classroom.
The instructions will give more detail, but the teacher arranges for someone to come into the classroom while the teacher steps out for a minute. While the teacher is away, the "criminal" (usually another teacher) does a few mischievous things like drawing on the white board, taking a backpack, etc.
Once the criminal leaves and the teacher returns, the students will then fill our a police report about the indecent using the imperfecto to describe and the pretérito to tell the actions that took place.
This activity assumes that the students have prior knowledge of the pretérito and imperfecto. It is most suitable to intermediate and higher students.