A、We always use body language to express our feelings. B、Body language can be controlled by emotions. C、Emotions will be express

Any student of body language knows that there is one universal truth: Whether we wish to or not, we communicate what we feel. We may keep our mouths shut, but our bodies inevitably find other ways to make our emotions known.(19)In this kind of language, this principle is called " leakage"; the involuntary betrayal of emotions through body language and gestures.
    We play with a simple version of this principle as children. In the game "Hot or Cold," one player hides an object in a room, and the second player moves around the room, asking whether he is "hot" or "cold." The behavior of the hider, who is sitting on the sidelines, is telling. Some hiders carefully look everywhere but at the hidden object, while others secretly flick their eyes in that direction, as if to reassure themselves that it is still safely stored. With still other hiders, it’s not so much the eyes as the body—every movement is telling. If you’re watching the hands, the eyes and the feet of the hider, you’ve got a real map.(20)The object can be easy to find, because the person who hid it has been silently shouting its location the entire time.
    In trying to understand the evolutionary purpose of depression, scientists like psychiatrist Andy Thomson and evolutionary psychologist Paul Andrews have discovered that(21)depression may function in the same way as leakage. When a person represses his true feelings, depression can serve both to signal the repression and point the way to the hidden object that is causing the pain.
19. What is the principle of "leakage" mainly about?
20. Why could the hidden object be easily found in the game?
21. What do scientists discover about depression?