The Birthday Door

The first time Elena saw the shimmer in the mirror, she thought it was a trick of the light. She was six, sitting cross-legged on the carpet of her small bedroom, the soft, uneven beams of the afternoon sun spilling through the curtains. Her mother had left her alone with a slice of cake and a candle, humming quietly as she washed dishes in the kitchen. It was her birthday. Their birthday.

Elena had always known she was supposed to have a sister. Her mother never hid it from her. “You were born first,” she’d said, “and your sister… she didn’t make it.” She said it gently, with a sadness that felt older than Elena’s six years, and Elena would nod, because she could feel it too. A presence. A missing piece.

That day, as the candle’s flame flickered and wax dripped onto the plate, Elena felt the air shift. The room smelled faintly of rain even though the sky outside was blue. She looked at the mirror propped against the wall — a cheap, tall rectangle with silvering gone at the edges — and there it was: a shimmer, like water disturbed by a fingertip.

And then she saw her.

The girl in the mirror had the same small, heart-shaped face. The same dark hair falling into her eyes. The same dimpled chin. Only her dress was different — pale blue where Elena’s was yellow — and her eyes… her eyes glowed faintly, as if she’d been standing in a sunlit meadow.

Elena blinked hard. The girl blinked back, but not at the same time.

“Hi,” the girl whispered. Her voice was faint, like an echo traveling through a long tunnel. “I’m Isla.”

Elena’s heart thumped once, hard, then softer. She didn’t scream. Somehow, she wasn’t afraid. “I know you,” she said.

“I know you too.” Isla tilted her head, smiling. “It’s our birthday.”

Elena crawled closer to the mirror, pressing her palm against the cool glass. Isla mirrored her. Where their hands met, the surface rippled and grew warm, as though a thin sheet of water separated them.

And then, for just a moment, Elena felt fingers — warm, real fingers — against hers.

She gasped. The mirror went still. Isla’s image flickered, then steadied.

“Same time next year,” Isla said, her voice stronger now. “Same place.”

And she was gone.

For a year, Elena kept the secret. She thought about Isla constantly — what her life might be like on the other side of the mirror, what her house looked like, what her mother was like. She began to sense things, faintly. Sometimes at night, when she closed her eyes, she’d see flashes: a green bike leaning against a fence; a dog with one blue eye and one brown; a kitchen painted yellow instead of white.

Her mother noticed the change. “You’re quiet today,” she’d say. Or, “What’s on your mind, sweet pea?” But Elena just shook her head. It wasn’t something she could explain. Not yet.

The next birthday came with a storm. Rain lashed the windows, and thunder rolled across the hills. Her mother had baked a chocolate cake, the scent rich and comforting, and after the candle was blown out, she’d left Elena to play.

Elena went straight to the mirror. She didn’t even have to wait. The shimmer appeared instantly, like a curtain being drawn back.

Isla was taller now, her hair longer. She was wearing a green sweater, and behind her, Elena glimpsed a room full of balloons.

“You came back,” Isla said, her voice bright.

“I promised,” Elena said.

They pressed their palms together again. This time, the surface gave way like warm honey, and Elena felt Isla’s hand completely — small, soft, trembling.

Elena’s breath caught. “It’s real,” she whispered.

“I know.” Isla grinned. “I knew you’d find me.”

They sat like that for a long time, separated by a sliver of worlds, telling each other everything they could think of. Elena learned that Isla lived in a house that looked almost exactly like hers but faced the opposite direction, as though someone had flipped a photograph. Their mother was alive there too, but different — younger, laughing more, her hair longer. Their father was there, too, though Elena’s father had left before she could remember him.

Elena asked if Isla knew why they could meet. Isla shook her head. “It only happens today. Our birthday. It’s like… a door that opens. But just a crack.”

As the years passed, their meetings became ritual. Every birthday, Elena would close her door, sit cross-legged before the mirror, and wait for the shimmer. It always came. Sometimes they just talked. Sometimes they drew pictures together, pressing crayons on either side of the glass. One year, they tried to pass a necklace through, and it worked — a thin silver chain slipping from Isla’s palm into Elena’s. Elena still wore it every day, hidden under her shirt.

But as they grew older, the meetings became heavier. At twelve, Isla confessed that she’d started having dreams — dreams of Elena’s world. “I see you walking to school,” she said. “I see your friends. Sometimes I even hear your mom’s voice.”

Elena had been having the same dreams. “Maybe we’re the same person,” she whispered. “Split in two.”

“Maybe,” Isla said. “Or maybe we’re supposed to be together.”

They both looked at the mirror then, at the thin, watery barrier between them.

At fifteen, Elena’s world began to tilt. Her mother remarried, and her stepfather didn’t like closed doors. He knocked once and entered without waiting, and Elena nearly screamed when he found her kneeling before the mirror, holding Isla’s hand.

But to him, the mirror was just glass. Isla wasn’t there.

After that, Elena became secretive. She moved the mirror into the attic, the only place no one ever went.

When their sixteenth birthday came, she found Isla already waiting. She looked pale, her eyes shadowed.

“Elena,” she said urgently. “It’s getting harder.”

“What is?”

“This.” Isla pressed her palm to the glass, but the ripple was weaker. “It’s like… the worlds are pulling apart.”

Elena’s stomach clenched. “No. We can’t lose this.”

“We might not have a choice.” Isla’s eyes filled with tears. “But I think — I think there’s a way. Next year, when the door opens again… if we both want it badly enough, maybe one of us can cross over.”

Elena stared at her. “Cross over?”

“Be together. In one world.”

The attic creaked in the wind. The candle flickered between them. Elena felt her heart hammering. “Which world?”

“I don’t know,” Isla whispered.

That night, Elena couldn’t sleep. She lay awake, staring at the ceiling, the necklace warm against her chest. She imagined Isla lying in her own bed, mirroring her. She imagined what it would be like to live in Isla’s world, where their father had stayed, where their mother still laughed. Or to bring Isla here, to the world where she’d been born alone.

A terrible thought struck her: maybe one of them would have to give something up. Maybe one of them would have to disappear completely.

She pressed her hands to her eyes until she saw stars.

On the morning of their seventeenth birthday, the sky was white with fog. Elena went to the attic early, carrying a single candle. She placed it on the floor and lit it, the flame trembling.

The mirror shimmered before she even touched it.

Isla stood there, her hair loose around her shoulders. She looked more real than ever, as though she were already halfway through.

“Elena,” she said, her voice low. “It’s time.”

“Time for what?”

“To choose.”

The room seemed to shrink. The air grew heavy. Elena knelt, pressing her palm to the glass. “I don’t want to lose you.”

“You won’t.” Isla’s hand met hers. The surface between them glowed faintly, like the skin of a bubble catching sunlight. “We’re the same. We’re just… waiting to be whole again.”

Elena felt heat rushing up her arm, through her chest. The necklace burned against her skin. The glass dissolved between them, and suddenly she was holding Isla’s hands, real and warm, no barrier at all.

For a moment, they both gasped.

“It’s working,” Isla whispered.

Then the attic around them flickered. Elena saw a kitchen painted yellow. She saw a green bike leaning against a fence. She saw two mothers, two houses, two skies.

“Elena,” Isla said, her voice breaking. “You have to decide.”

“I don’t understand—”

“One of us has to go.” Isla’s eyes were full of tears now. “Only one of us can stay.”

Elena’s heart cracked. “No. We can both—”

“We can’t.” Isla shook her head fiercely. “If we try, we’ll both be lost. We’ll both disappear.”

The flickering grew faster. Elena felt herself being pulled, as if by a tide. She clutched Isla’s hands harder.

“Choose,” Isla said. “Please. Choose.”

Elena thought of her mother downstairs, humming. She thought of Isla’s world, where her father still sat at the dinner table. She thought of growing up alone, always missing someone, always feeling incomplete.

And she knew.

She pulled Isla forward, hard, into her arms. The world went white.

When the light faded, she was lying on the attic floor. Her hands were empty. The mirror was just a mirror again — plain glass, no shimmer, no ripple.

Footsteps sounded on the stairs. Her mother appeared in the doorway, looking startled. “Elena?” she said. “You’re up here?”

Elena sat up slowly. Her heart was still hammering, but the weight in her chest was gone.

Her mother stepped closer, frowning. “Are you okay?”

Elena opened her mouth to answer — and stopped.

Because she knew, in a way beyond words, that she wasn’t Elena anymore.

She was Isla.

And this was her world now.

Her mother knelt and hugged her. “Happy birthday, sweetheart,” she whispered. “Seventeen today. My miracle girl.”

Isla closed her eyes, feeling tears slip down her cheeks. Somewhere, far away, she felt a flicker — like a hand brushing hers through glass.

“Elena,” she whispered, too softly for her mother to hear. “Thank you.”

That night, she dreamed of a mirror. On the other side stood a girl with her face, smiling through tears.

And Isla smiled back.

For the first time in her life, she didn’t feel the eyes on her back. She felt whole.

But she also knew: next year, on their birthday, the shimmer would return. And maybe — just maybe — the door would open again.

Because some bonds don’t break, even across worlds.