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Fixed points in time are events that are so crucial to the integrity of the universe's timeline that they must happen and cannot be prevented. One can alter the details of a fixed point to a very small degree (as seen in "The Fires of Pompeii" and "The Waters of Mars"), but the event itself has to happen. If a fixed point is prevented from occurring, then it will cause time itself to slowly collapse (as seen in "The Wedding of River Song"). Additionally, fixed points can be created from "still points" in time, but what exactly a still point in time is is unknown.