The Faculty of Dentistry smelled of mint, eugenol and warm plastic, as it always does after a day of autoclaves. The corridor was illuminated by cold lamps; 'Night of Healthy Smiles' posters waved quietly from the draught. On the tables, sonic and manual toothbrushes, dental floss, braces wipes and small leaves with instructions were evenly arranged: twice a day for two minutes, fluoride toothpaste 1450 ppm, floss once a day, habit instead of compulsion. Lena looked out from behind the curtain of Cabinet 3 and brushed back her hair in a pony tail. She could feel the freshness in her mouth from the rinse, but the timer set for two minutes was still buzzing in her sweatshirt pocket, a habit that had turned into a reflex. She slid her teeth down the outer wall of the clear aligner - the thin plastic arches clicked against the enamel - and tucked them into the case. Without them, her smile seemed half a tone different, more youthful, less trained. - Ready? - Nikolai floated out from behind the door of the radiology lab, his smile gleaming metallically. His fixed braces shone like a fine mechanism - bows, brackets, ligatures. He had a portable irrigator by his belt, from which barely a drop dripped onto the floor. - 'Before the guests get busy, I'll show you something,' called out Dr Zofia Wierzbicka, the wheel's caretaker, slim and always impeccably calm, although today she had a sparkle in her eyes as when new gadgets arrived. She lifted a small camera with a filter from her trolley and pointed it at a model of a jaw standing on the tabletop. - Biofilm. Under this light it glows like a purple haze. Not to scare anyone. The idea is to see what is normally invisible before it turns into trouble. Lena liked how, in Dr Sophie's voice, 'problem' became 'trouble', something that could be overtaken and controlled. She didn't know another person who could talk about plaque like a cloud that could be dispelled with breathing and brushing. The evening rolled on with the rhythm of the demonstrations: students shone lamps, the crowd spun between offices, children snapped pictures of their teeth with smartphones, adults asked about fluoride and whether an irrigator could replace floss. Lena and Nikolai helped - she explained how to guide the toothbrush under the gum line, he demonstrated the picks on miniature models of locks. Every now and then, they would peek into the bathroom to wash off any leftover plaque-staining tablets from their mouths, which they handed out to the curious. When the crowd had thinned out a little, Dr Sophie nodded for them to come up: - Try it on yourselves. To know what the visitors see - she handed them each half a staining tablet. - Then wash right away. Don't cheat on the time. The tablet dissolved sweetly and sourly. Nikolai laughed soundlessly as his tongue was already dark blue. Lena spread the dye with the tip of her tongue over all her teeth, as instructed. Then they stood in front of the camera. On the screen, Lena saw her own smile as if in another galaxy. Where the toothbrush reached exactly, the enamel was clean, neutral; in the nooks and crannies near the molars, purple shimmered. At the incisors on the cheek side, where she usually brushed, tiny clouds appeared. There wasn't a lot of it, but to see one's habits under ultraviolet light was like getting a note card in the margins from a mirror. - Not bad,' winked Dr Sophie. - 'And you, Nicholas? In Nikolai's photo, tiny spiral swirls blossomed alongside the usual 'shadows' around the locks and under the arch. They were like miniature galaxies glued to metal surfaces. They arranged themselves in a rhythm that Lena couldn't name, and yet... she knew it. - Can you see it? - She asked, lowering her voice. - That motif. Like on the ramp at the skatepark, that spray in the shape of a snail. - Really? - Nikolai squinted his eyes. - 'Maybe it's the usual flashes. After all, there's always something left by the locks. They went to brush their teeth. Lena set a timer, spread fluoride toothpaste with the soft tip of her toothbrush and moved it over the chins of the molars in small, clockwise circular motions. She didn't scrub, just swept under the gum line. Then the floss - a loop, gently, with feeling, Dr Sophie said - between the teeth until the 'click' came off with resistance. Nikolai stood beside him, armed with a toothpick and patience worthy of a surgeon. He smiled at the mirror so broadly that it hissed. After washing and rinsing, they returned to the camera. The spirals by the camera had faded, but a few remained, as if someone had put them not on a plate but on metal. - 'If the author of the ramp is using paint doped with a fluorescent pigment, it may reflect the light just like the dye,' Dr Sophie muttered, to herself rather than to them. - Or someone from us is playing with the tests outside the building. Lena looked at the screen and felt a slight shudder. She was practical. Her world consisted of calendars, schedules and reminders. And yet there were moments when things came together in an unusual pattern, like drops of water coming together to form a drawing on the glass. They left the building before eleven o'clock. The air was crisp, drenched in sea chill and the distant noise of the railway. - 'I'll walk you to the bus stop,' Nikolai said, slinging the UV lamp over his shoulder. He'd been given it by Dr Sophie "for a while to bring back to the depot", but they both knew that a moment can be prolonged when it's a Saturday outside and something inside is pulling like a bar. In the square in front of the department, just off the bench, someone had been playing with chalk earlier. White snails of spirals, barely visible after the rain, stretched towards the fountain. Nicholas, out of sheer spite, turned on the lamp. The chalk lit up milky. But not the chalk. Between the lines, where the surface was darker, narrow streaks bloomed, as if someone had dragged a toothbrush bristle soaked in something UV-loving across the stone. The sweet-mint smell hit them almost simultaneously. - 'Paste,' Lena said. - 'Of those with optical brighteners. See how it reflects? Someone spread it here on purpose. The footprints trailed like a wolf's track in the snow, the direction marked with little arrows of dark blue dye that was almost invisible under ordinary light. Lena lifted her rucksack higher on her shoulder, looked around. The square was empty. The lanterns looked like slender teeth, their light like enamel under a lamp. - 'Let's walk a bit,' Nicolas suggested. - I'll bring the lamp back right away. Let's see where it leads. Maybe it's a game prepared for today by someone from the year? A kind of... test of perceptiveness. - Who would use paste and dye for a game at university? - parsed Lena, but curiosity began to push reason apart. - Just a moment. And we don't touch anything that's not ours. They moved along the quay. The trail passed a kiosk selling late-night buns (lots of sugar, Lena thought, and then answered herself: don't be stereotypical, it's the frequency that counts, don't demonise). It led across the square, under the murals, past the skatepark. On the ramp, where someone had left a spiral graff a week ago, a UV lamp brought out a thin fastening of letters from the background - individual fibres of shadow, phonetic needles connecting the outlines. The letters formed the inscriptions: "Rinse when there is no water", "Thread is a bridge", "Glaze remembers". - 'Someone has really put their mind to it,' whispered Nikolai, no longer joking. - And he knows his stuff. The footsteps plunged again into the darker part of the park, where the trees scraped their branches against the sky. There, under the amphitheatre, was a service door that everyone knew about, but no one took seriously. A steel plate, a grille with a circle cut out for a handle. To the eye it was locked to the amen. The UV lamp caught something the naked eye couldn't see: on the door, in a thin, dry layer, someone had written with a brush dipped - Lena caught the smell, barely graspable - in a wash of peppermint oil. A molar sign with an arc drawn inside. Underneath in smaller letters: "Sana dentes, sana anima". - 'It's a joke,' Lena whispered, but she flipped the lamp onto her left palm to keep her right hand free. Her heart rattled under the collar of her blouse, as if someone had set the internal metronome to a little too fast allegro. Nikolai put his fingers to the handle wheel. The metal was cold. Someone had written near the hinge, so fine it was barely visible: "Smile when you see the light". - You know it could be some kind of installation,' he said in a half-hearted voice. - Or maybe a camera. Or nothing at all. - Or someone wants to show us something that only clear enamel will see,' Lena replied, not even sure where she got that from. She touched her tongue against a molar; she distinctly felt the fresh, cool aftertaste of the rinse. After all, she had just brushed her teeth. She lifted the lamp. A purple stain trembled at the edges of the signs. A sheen of some sort flashed through a crack in the door where the paint had come off. Unpleasant like the glint of a knife? No, not metallic - glassy, like shattered enamel... or a mirror. There must have been movement somewhere inside, though, because suddenly, completely silently, the lock mechanism clicked. In the silence of the night, the click sounded like a gunshot. Lena's breathing became foggy. A chill blew from inside and mint invaded her throat, intense and pungent, as if someone had unrolled the hose from an irrigator here and just closed the valve. A blue light pulsed in the gap, as if someone inside had turned on a polymerisation lamp and covered it with their hand. - 'Do you hear that? - Nikolai whispered. The door vibrated a second time, as if on the other side someone was gently, kindly, encouraging them to enter.