An October drizzle coated the windows of the former clinic in Hospital Street the way a soft film settles on teeth after too sweet a tea. A pulsating smile-shaped neon sign winked above the entrance: once sharply, once subdued, as if it wasn't sure itself whether it still had anyone to shine for. Inside, it smelled of latex glove powder, eugenol and freshly ground coffee from a tiny coffee machine, which Lena had turned on reflexively before taking off her shoes. Turning on the white light above the dentist's chair cut the twilight with a precise cone, like a beacon. Lena liked this silence after hours - everything here had a zero position: drawers closed, wires coiled, instrument tray covered with a sterile shawl, like a dormant piano. She spent her entire childhood in this place, learning to count on the shiny tips of the mirror, peering into the sample centrifuge, sitting on the rotating stool and swaying as her mother repeated to her patients: "Two minutes, twice a day. And floss. You become friends with the floss, like a favourite song - you return to it every day." Today was supposed to be scientific, not sentimental. Lena was in her graduating class and was preparing a project for Scientists' Night. The topic sounded dry: the effect of vibration frequency on the behaviour of oral biofilm. But what she saw in the school bathroom yesterday - turquoise, tiny flashes on her teeth under the UV light from the theatrical test lamp - made her feel a stab of curiosity like an invisible finger between her shoulder blades. Her peers joked that it was the "selfie effect", that PopPearls' new candy was "tinting her smile on trend". Aga sent pictures: delighted, with the pink wrapper in her hand. Lena didn't like it when something shines in her jaw other than enamel. She rolled up the sleeves of her blouse and washed her hands, counting down to thirty in her mind. She put on her gloves, turned on the autoclave for the control test - a habit - and set her mug of coffee far from the microscope. On the lab counter, she prepared basal slides. One with a sample of her own biofilm from around her incisors - captured with dental floss, which she pulled securely without hurting her gingival papillae. The second with smugness: tiny scrapes from the surface of the sink filter by the spittoon, which she had cleaned in the morning. The third with the shards of PopPearls dissolved in a drop of saliva that Aga had left for her in the cupboard, the packet described in pastel screams - zero sugar, zero guilt, zero boredom, nano-shine. The phone vibrated. Kacper: "Are you coming over to the bonfire?" Lena swiped the message, glassily indifferent. Not today. Today the sounds had a different meaning. She set the sonic toothbrush next to the microscope, the same one she used in the mornings - a clean head, with clear, untrimmed bristles. She liked the timer - four quarters, thirty seconds each. She also liked the knowledge that the toothpaste had fluoride and that, rather than rinsing straight away, it allowed the mineral lozenges to stay in her saliva for a while. It was like leaving a good song to ring out to the end. Under the microscope, the edge of her own biofilm looked like seaweed in shallow water, vibrating slightly from the heat of the lamp. She set the magnification to 400x and thought for just a second that it was intimate: the highways between enamel and gum that only she knew about. Then she switched on the toothbrush, but put it a metre away so as not to transfer moisture. The sound, a resonance of 31 kHz, rolled like a breath across the tiles. And then, as if on command, the thin threads of biofilm under the glass began to draw non-accidental lines. The stubborn randomness of the bacterial wanderings died away. They spread out like patches of grass, opening an arrow-shaped clearance. The arrow did not point into the void - it pointed to the side of the table, towards the spittoon. Lena straightened up, blinking as if to flush her eyelid with her tears and thus clear the image of her own projections. She switched off the toothbrush. The threads returned to rippling. She switched on the 42 kHz mode. This time the needles of nanoparticles from the PopPearls under the second glass moved like a flock of shoals in the current, tracing a honeycomb pattern, perfectly even. Too even to have been made by accident. This pattern also aligned itself, like a springy compass needle, in the direction of the water system. From the depths of the room, a compressor flickered, like a sluggish dog shifting its position on the carpet. Lena smiled almost reflexively, but the muscle of that smile did not reach her eyes. She pulled her hair into a tight ponytail and crouched by the filter cabinet under the armchair. The metal casing was cold, the surface greased with the fingers of hundreds of reviews. With a French spanner she loosened the ring. The filter was heavier than she remembered the last time she had changed it with her mum; it was sloshing inside. Sticking out of the side was a tiny, almost transparent micro-print sticker. She flicked the torch over. The letters had gone grey at the edge of visibility: SGRLP-PP. The phone vibrated a second time. A new notification: unknown sender, yellow dot. "Do not turn on 42 kHz." Lena, usually one to plough straight through barrages of secret messages, felt a cooler wave pass through her spine. She looked reflexively into the lens of her laptop, sealed with black tape. Stupid. She slipped a gloved hand over her fingers to keep the sweat on her palm from shattering her composure. The sound of the autoclave's toothed belt began to tick rhythmically. The old fluorescent light in the corridor to the storeroom blinked. Someone in the street walked by, the conversation dimmed, the crunch of gravel beneath her shoes moved away. Lena opened the filter halfway and stopped, sniffing. A minty smell, but metallic, like from a teaspoon. A memory: the chemistry room, benzene, the smell of Aga's hairspray. She looked at the PopPearls packaging - zero sugar, zero calories, zero gels, zero fluoride, active NANO-GLEAM complex. For chewing without restriction. She put her own mirror to her lips and switched on the small, purple UV torch that was usually used to detect plaque deposits in patients as part of hygiene instruction. The shadow in the bathroom lamp showed this yesterday too. Now, in the sterile light, she saw turquoise sparks in the crevices of the furrows, like a glare under the ice. She moved the dental floss, slowly, consistently, feeling the muscle strings against her cheeks straighten under control. A disciplined movement. The plate came away like a spider web. The shimmer did not disappear. It was no ordinary colour. Something had entered the enamel like a fine thread in a knitted fabric. Mum always said: oral flora are not enemies, they are neighbours - they need to be kept in check, not bombarded. Fluoride acts as a protective mantle for the hydroxyapatite, it strengthens it, it doesn't do wonders on its own. Lena felt this lecture in her muscle memory like a rhyme. Therefore, a project about frequencies seemed ideal: sound as a broom or as a melody for order. Only that the sound here brought her back to the filter, as if something was waiting there, programmed to respond. She returned to the cabinet and slid the filter all the way out. Inside, behind the first sediment mesh, was a transparent capsule pressed like an alien seed into a fruit. It was filled with a suspension of tiny globules shimmering under the UV torch. The same, intrusive shade of turquoise. Lena touched the capsule with the tip of her tweezers. The globules twitched, clumped together in a ripple, then seemed to flatten along a single line - the same line the biofilm had recently indicated. The smile of the neon sign above the entrance dimmed; for a second there was absolute silence, then the old compressor sighed again, emitting a tone that, if you let your imagination run wild, formed something akin to a whisper. "Lena..." She moved her hand away abruptly. It was a resonance in the pipes. It had to be. Before she had time to laugh at herself, the sonic toothbrush on the countertop switched on the mode itself. She didn't touch it. The vibrations passed smoothly over the countertop, as if someone had dragged a finger along the rim of a glass. The first slide under the microscope, left in the light, showed a movement so coordinated that it was beautiful and disturbing at the same time. The transverse bands of biofilm receded like blinds under the touch of his hand. The pattern under the second sample - with PopPearls - formed hexagonal planes like snow crystals, except that each plane vibrated in a rhythm she could not feel in the pulse of her wrist. Aga wrote: "Oi, and if my gums bleed a bit after this new rinse, is that normal? XD." Lena immediately wrote back: "Put down the rinse and talk to your hygienist or parent tomorrow. Seriously." She added a heartbeat to avoid sounding like a lecture. Three blinking answer dots appeared on the screen and promptly disappeared. The phone rang again. The same stranger: "If you breach the layer, you will release them." There were no emoticons, no dots. Just the tone. You will release them. She shrugged her shoulders, but felt a familial tightness in her throat. She remembered the lab warnings about filter pressure. Okay. Gloves? There they are. Goggles? On her forehead. She slipped the mask over her face, the voice inside sounding strangely her own because of it. She set the brush timer for two minutes, as she always did when she wanted to collect her thoughts: two full circuits of breath, thirty seconds for each quarter. The digital clock on the stand chimed 2:00 a.m. She slid the tweezers under the edge of the capsule. The coffee on the other side of the countertop cooled, its smell becoming flat, unhelpful. The water in the spittoon suddenly moved, though no one touched it: as if someone had blown on the surface. The suction hose trembled. She heard quick footsteps in the stairwell - was it someone? Or was it just her own sound reverberating off the marble? The neon blinked shorter. In the storeroom, some kind of plastic tray had been knocked over; the echo was longer than it should have been. Lena clenched her teeth, sliding her tongue along her incisors - a simple, reassuring movement, like checking the boundaries. Then she pressed her thumb against the joint line of the capsule. A drop of water from the wet hose fell acutely onto the transparent surface. The balls inside trembled as if someone had pressed play. The sound of a toothbrush rustled through the air, much lower, like a buzzing under the skin. When she pulled gently, the thin shell gave way with a soft crackle - the same second something turquoise moved in the spittoon and took on a shape that should never have formed there.