"At some point she found herself remembering the year in early childhood when she had gone around the house looking into mirrors.
She would stand in front of them with her hands cupped to the sides of her face and her nose touching the glass, but holding her breath so she wouldn't fog the surface.
If her mother caught her, she'd swat her away. That leaves a smudge, and I have to clean it off. Why are you so interested in yourself, anyway? You'll never be hung for your beauty. And why stand so close? You can't see anything worth looking at that way.
How old had she been? Four? Five? Too young to explain that it wasn't her reflection she was interested in, anyway--or not primarily. She had been convinced that mirrors were doorways to another world, and what she saw reflected in the glass wasn't their living room or bathroom, but the living room or bathroom of some other family. The Matsons instead of the Madsens, perhaps.
Because it was similar on the other side of the glass, but not the same, and if you looked long enough, you could begin to pick up on some of the differences: a rug that appeared to be oval over there instead of round like over here, a door that seemed to have a turn-latch instead of a bolt, a light-switch that was on the wrong side of the door. The little girl wasn't the same, either. Darcy was sure they were related--sisters of the mirror?--but no, not the same. Instead of Darcellen Madsen that little girl might be named Jane or Sandra or even Eleanor Rigby, who for some reason (some scary reason) picked up the rice at churches where a wedding had been.
Lying in the circle of her bedside lamp, drowsing without realizing it, Darcy supposed that if she had been able to tell her mother what she was looking for, if she had explained about the Darker Girl who wasn't quite her, she might have passed some time with a child psychiatrist. But it wasn't the girl who interested her, it had never been the girl. What interested her was the idea that there was a whole other world behind the mirrors, and if you could walk through that other house (the Darker House) and out the door, the rest of the world would be waiting.
Of course this idea had passed and, aided by a new doll (which she had named Mrs. Butterworth after the pancake syrup she loved) and a new dollhouse, she had moved on to more acceptable little-girl fantasies: cooking, cleaning, shopping, Scolding The Baby, Changing For Dinner. Now, all these years later, she had found her way through the mirror after all. Only there was no little girl waiting in the Darker House; instead there was a Darker Husband, one who had been living behind the mirror all the time, and doing terrible things there."
(source for quote: Full Dark, No Stars by Stephen King, published by Scribner, New York, NY © 2010.)
The English Beat - Mirror in the Bathroom