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In the end of Avengers: Endgame, Tony Stark somehow
manages to pull a "switcheroo" on Thanos, ending up with the Infinity Stones on his own Iron Man gauntlet rather than the one Thanos is wearing.
This is kind of an important moment--the entire climax of the film hangs on it!--but I don't believe it was ever shown or explained how he did it. I can think of a handful of ways how he might have done it, especially if he had had the foresight to set it up as a possibility from the beginning, but I didn't notice any of them actually being shown. Did I miss something, or did that happen "just because" with no explanation whatsoever?