Room 101 is deep underground. Once inside, Winston is immobilized and strapped to a chair. O'Brien tells him that Room 101 contains "the worst thing in the world," and that this thing varies from person to person. O'Brien lifts up a cage containing two huge, starving rats and a kind of door that can be fitted right up against Winston's face. O'Brien brings the cage nearer and tells Winston that when he puts the cage in place and opens the door, the rats will devour his face. As the cage and rats come closer and closer, Winston is overwhelmed by a black panic. He knows that the to save himself he must place a body between himself and the rats—and that that body must be Julia's. He shouts frantically, "Do it to Julia! Not me!" O'Brien, pleased, removes the cage.
Room 101 contains a person's greatest fear (which the Party learns through constant surveillance), but it also contains an even more horrible lesson. The room teaches Winston that in the face of his greatest fear, he would be willing to sacrifice anything–love, dignity, loyalty–in order to escape that fear. Put another way, he is willing to sacrifice everything that makes him himself, his own spirit and heart, to save himself from the thing he fears. Through that fear, O'Brien and Party have taken control of Winston's mind, and he will never die as an individual..