I *think* this is a Ray Bradbury story I read 15-20 years ago. The main characters were a pair of twins who discovered a time warp in a shed in their backyard. Time passed more quickly inside the shed than in the rest of the backyard. The twin who was slightly younger (and had always felt inferior) decided to spend the night in the shed (which turned out to be a year of elapsed time inside the shed) so that he could be older than his twin. While in the shed, he worked out a lot so that when he emerged, he looked remarkably different than his brother. There was also a mirror or something in the shed in which he could see an object getting larger day by day. I believe the story ended abruptly, implying that something disastrous came through the mirror.

Does anyone recall this story? My sister thinks it might have been called The Door into Tomorrow or something, but Googling that has not turned up anything.

Thanks!

AHA! Now, THIS one, I actually KNOW! "Singularity," by William Sleator. Popular among the stumper set, though surpassed in number/frequency of requests by Sleator's "House of Stairs." Sleator also wrote "Green Futures of Tycho" and other intriguing-sounding books. Sounds like an author well worth looking into!