What would a robot have to be able to do, or what would it have to be, for us to

What would a robot have to be able to do, or what would it have to be, for us to consider it a sentient being as opposed to a non-sentient automaton? Please note I am using the term "robot" here in a broad sense, including such obviously sentient (fictional) constructs such as C-3PO of Star Wars fame. I don't consider "robot" and "sentient being" to be mutually exclusive terms. I'm interested in what fundamentally distinguishes sentient beings from automatons that merely mimic sentience.

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