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"What If—" is a fantasy short story by Isaac Asimov that was first published in the Summer 1952 issue of Fantastic and reprinted in the 1969 collection Nightfall and Other Stories. The story was inspired by the author's wife, who challenged him to produce a story based on a train trip they were taking to New York.
A married couple is traveling on a train from Boston to New York City. They meet a mysterious silent man known only as Mister If, who shows them a small portable television-like device about 6 by 9 inches (15 cm × 23 cm) in size. (Mr. If's first name is implied to be What, but whether this is true or not is left to the reader.)
On the device's screen, Mr. If shows the couple scenes from their earlier lives and what might have happened if certain minor but pivotal events had not occurred.
Similar devices in subsequent fiction
The story's theme and idea has been used in two episodes of the animated television series Futurama: "Anthology of Interest" and "Anthology of Interest II". Professor Farnsworth demonstrates a 'What If—' machine similar to Mr. If's.
Neil Gaiman wrote a short story called "The Wedding Present" as introduction for the collection "Smoke and Mirrors". In this story, a couple receives as a wedding present a stack of paper sheets relating what could have happened during their to-come life in common.