A proof 证明
A man came to a village post office for a registered letter. The letter was there, but the clerk did not give it to the man. "How can I know that the letter is for you?" he asked.
The man took a photo of himself from his pocket, and said, "I think now you know who I am?"
The clerk looked long at the photo and then said, "Yes, that is you. Here is your letter."