At secondary school, one of my classmates lost her locker key.  During a class I saw two other classmates holding a key and talking secretly.  I do not know why I suspected them, also another classmate supported me.  He remembers that these people had only one key in the key-ring, this had seemed strange to him.  Besides another classmate said that he had seen them selling books at a book-shop and they should not have had such books.  I told the teacher and the person was caught.  He had sold the books and was forced to quit school.  I found that I had not helped him as he stole later on also.  I felt guilty