When the reason for a slight gain in weight is not immediately evident, it is necessary to investigate further. A patient who is unaware of having committed an error or is unwilling to admit a mistake protests indignantly when told he has done something he ought not to have done. In that atmosphere no fruitful investigation can be conducted; so we calmly explain that we are not accusing him of anything but that we know for certain from our not inconsiderable experience that something has gone wrong and that we must now sit down quietly together and try and find out what it was. Once the patient realizes that it is in his own interest that he play an active and not merely a passive role in this search, the reason for the setback is almost invariably discovered. Having been through hundreds of such sessions, we are nearly always able to distinguish the deliberate liar from the patient who is merely fooling himself or is really unaware of having erred.