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When a robot almost looks human--almost, but not quite--it often comes across as jarringly fake instead of familiar. Robots that are clearly artificial, like WALL-E or R2-D2, don't have this problem. But androids like this one that imperfectly mimic human mannerisms and facial expressions are weird enough to be haunting. This phenomenon is known as the uncanny valley. It's a major obstacle for designers who try to make their robots look like people--and it may be just as much of a hurdle for developers who are creating bots that talk like people, but that don't have a body at all.