The old building of the Department of Dentistry smelled of steam, eucalyptus and metal. The night was quiet, so quiet that even a drop of water falling from the tip of a turbine sounded like the impact of an empty cup. Maja entered the dental hygiene laboratory, switching on only one lamp above her desk. The corridor stretched behind her back like a long tongue of a building that had fallen asleep but had not stopped listening. She carried her rucksack, a mug of the now cool sage and a book for her preventive medicine exam. She was in her third year. She already knew the smell of chlorhexidine, the rustle of gloves, the dull sheen of the tiles on the phantoms. Yet the night duty had something special about it - the whisper of the filters in the units, the steady hum of the compressor in the basement, the echo of her own footsteps bouncing off the ceramics. On the blackboard under Dr Wolf's framed diploma, someone had left an old note: "Never confuse strength with diligence". Maja smiled under her breath. The soft bristles of a toothbrush - that was the point. Someone had once added in pen in the margin: "Two minutes matter". She moved her finger over the addition, as if checking that the ink had long since dried. Before she started work on the biofilm report, she went to the back room. Brushing her teeth at 11:40pm was her stubborn little routine, an anchor in a sea of tasks. She pressed the head of the soft toothbrush, squeezed out the fluoride toothpaste and turned on the timer. A circular motion, two upper quarters, then two lower quarters. She pressed gently, felt the mint foam disperse across her palate. Then the floss - she glided carefully between her teeth, a letter 'C' around each contact. Not painfully, just thoroughly, she repeated to herself. Finally, a tongue scraper, one, second gentle pass. She spat, rinsed her mouth with water and felt the tension in her neck let go a little, like a jar lid unscrewed too tightly. As she returned to the phantom room, she noticed a light on one of the desks. Who had left it on? The lights were usually switched on in the morning, in front of the students. The phantoms - plastic heads with acrylic teeth - stood at their stations, with their mouths open as if to make an endless "aaa". She put down her notes and walked over to the switched-on stand. The desk was marked number 7. Maja put on her gloves and blew on the mirror to wipe off the fine dust. In the light of the lamp, she could even see particles of air settling on the chrome. The phantom's head seemed slightly twisted. When she touched the cheek of the plastic, it was surprisingly warm, as if someone had worked on it a moment ago. - Kamil? - She called out as a precaution, hoping that the security guard had come to collect the sweets from reception again. All that answered her was the buzzing of the UV in the disinfection cupboard. Kamil was probably making his rounds on another floor. Maja slid the mirror between the acrylic incisors. Something glinted in the crevices on the contact surfaces - like a fine mist. Of course, the old fluorescent dyes could still glow for a while, but this... She shuddered. The streak was unusually even, forming a thin, almost calligraphic line. She moved the probe, gently, without pressure. The line did not disappear. - Did someone hurt you with force instead of diligence? - she muttered to the phantom, more to tame the silence than out of humour. She reached for the fluorescent plaque staining torch. She covered the stand lamp, turned off the main one and switched on the light with a cool purple hue. The phantom glowed with soft streaks - the places where biofilm usually builds up shone with a familiar glow. Only that on the incisors and canines, the strands were arranged in something that resembled a pattern, not a coincidence. As if someone had run a thread through the mouth with the precision of a watchmaker, leaving intentional marks. On the mirror hanging next to the desktop, someone had scribbled training slogans with a marker beforehand. Maja knew them by heart: "Two minutes", "Soft toothbrush", "Clean your tongue", "Use floss". That evening, next to the last one, she noticed a new addition, small and even: "Listen carefully". She moved her ear closer to the phantom. A sound? For a moment, she had the impression that she was hearing something more than a hum in her ears. A tinny buzzing, like the wings of a mosquito, like the too-distant signal of an ultrasonic scaler. No, that's ridiculous - the devices were off the grid. She glanced at the strip. The lights were going out blue. Everything was as dead as a textbook on holiday. Then she saw the thread. It lay on the floor, a narrow path of white sheen, as if someone had dragged dental floss across the room, leaving it on the cold tiles. It started at station 7, ran in a zigzag between the stools, disappeared under the sterilisation door. - Really? - She sighed, though her heart sped up. She picked up her head torch, put on her safety goggles - a habit. Though she felt silly, without a patient, without an assistant, at night. She opened the door to the sterilisation room. She was greeted by the coolness of the tiles and the smell of instrument cleaning fluid, sharp as pepper in her nose. On the shelves stood cassettes, trays of forceps, tweezers with leaves so delicate they resembled silver butterflies. The autoclave at the other end of the room was flickering with an LED - although, she remembered, the assistants had switched it off before she left. The thread trailed all the way to it, stopping at the door of the machine. Maja crouched down. The nylon shone like a hair. She put a gloved finger to it, touching the tip - it was cool, dry. She glanced at the cord. The autoclave plug lay on the ground, bent into a sad "U". Yet the display was flashing: 121°C, time: 00:07. - 'That's impossible,' she whispered, although she usually avoided the word 'impossible' by a wide margin in this job. She reached into her pocket for her phone. She texted Kamil: "Hey, are any of you playing with sockets? The autoclave shows it's working, but it's off." Dot blinked, then another. "I'll be right in. Probably a sensor. Don't touch." Don't touch. The word in the message overlapped with the word in her head. And the buzzing she felt again more than heard. She turned - behind the glass to the phantom room, the rows of heads still stood still, but for a moment she would have sworn that number 7 was turned a tad more towards her, like someone trying to hear a whisper from behind a wall. She focused her gaze on the door of the autoclave. Her own outlines were imprinted on the steel surface - a figure in a white apron, a headpiece like a crescent moon on her forehead, gloves of thin rubber. And just above her reflection, like breath on a mirror, a circular mark appeared. Fog? Steam? But after all, there couldn't have been moisture inside, since the device wasn't working. She turned on the timer on her phone for two minutes - a reflex that calmed her even when she wasn't brushing her teeth. Two minutes meant calm. Two minutes to decide. The bars of seconds moved like the beads of a rosary. - Okay - she whispered to herself. - 'We'll just check it's not a loose panel. No opening until Kamil comes in. Just the touch of the handle. Just a touch. As she brought her hand close to the cool metal, a sound like the quiet tipping of water in a cup came from behind the door of the main room - the one you hear when you rinse your mouth and tilt your head over the basin. Then a soft 'sssss' dripped down the tiles, as if a surgical suction machine had been switched on at minimum speed. Maja turned her head. In the light through the glass, she saw that fresh, damp writing had appeared on the mirror at post 7, as if etched in vapour in new, even writing: "Not by force". The bars on the timer stopped at 00:59. Her fingers touched the cool handle of the autoclave. At the same instant, a tinny, barely perceptible whisper rang out from behind the wall, which was neither a phone nor a compressor, and which was marked by a strangely familiar note - like the hiss of toothpaste as you squeeze it onto your toothbrush: - Don't touch.