A [[ScienceFiction sci-fi]] novel written by WilliamSleator for {{young adult}}s. It was first published in 1985.

The novel is about two twins, Barry and Harry, who semi-inherit[[note]]It technically belongs to their mother, but they go to check it out anyway.[[/note]] some property after their eccentric great-uncle dies. They discover that a metal shack (called the Playhouse) outside their uncle's home has a strange YearInsideHourOutside effect because something about that area acts as a portal to another universe.
!! This novel is an example of the following tropes:
* AlwaysIdenticalTwins: Barry and Harry, of course.
* [[spoiler: DarkIsNotEvil]]: [[spoiler: The scary-looking toothy thing is just coming through the PortalPool to destroy the portal to put an end to its dangerous effects, then eat itself alive before it can cause any harm itself.]]
* {{Diary}}: Harry keeps one while [[spoiler: spending a year inside the Playhouse]].
* IJustWantToBeSpecial: Harry. He doesn't want supernatural powers per se, but he wants to be different from his brother and to step out of his shadow. [[spoiler: He does.]]
* ImprovisedTraining
* MoreTeethThanTheOsmondFamily: The creature coming through a PortalPool from another universe. It was visible from this universe long before it arrived, and the only part of it that was visible was its huge mouthful of teeth. The main character, in fact, spent a whole year waiting for the creature to arrive, knowing only its toothy appearance.
* OutOfTimeOutOfMind: Averted. [[spoiler: The protagonist spends about a year inside a timewarp, which is about 8 hours outside.]] Changing his personality was his goal going in, and it works, [[spoiler: especially because the timewarp is only about 8 feet across, and he's the only person in it]].
* PlotRelevantAgeUp: Some cows wandered into the spot in the pasture where the Playhouse would be built and matured to old age overnight. Harry and Barry's uncle was also implied to have done this one night. [[spoiler: Also Harry does this.]]
* PolarOppositeTwins: Harry and Barry.
* PortalPool: When filled with water, the sink in the Playhouse shows images of another universe.
* RedOniBlueOni: Athletic, popular and pushy, Barry, is Red Oni to timid, book-smart and rational, Harry's Blue Oni.
* TwoGuysAndAGirl: Harry, Barry and Lucy.
* YearInsideHourOutside: Time on the "outside" passes extremely slowly while in the Playhouse.