Good for teachers. Tell your students that you are just going to note the scores that they have got on their tests or exams and will hand out to them after an hour. Go to the room and them keep running out looking like very scared and tell them that the principal has just spilled coffee or ink on the test/examination papers and they will have to take them again. Note their reactions and exclamations. You can bet that the dullest of all students will loudly claim that they had done their best this time and it is not fair to them. Then you can tell them how much they have really scored.