answer: #Person1# tells #Person2# #Person1# lost #Person1#'s backpack.
documents: #Person1#: Where is my back pack?
  #Person2#: I don't know. Where did you leave it?
  #Person1#: I just put it on the chair a while ago, but now it's gone.
  #Person2#: Are you sure?
  #Person1#: Of course, I am sure. I bet someone stole it.
  #Person2#: Well, you should have kept an eye on it.