Lena was fourteen, with braces and a stopwatch for brushing her teeth. Two minutes in the morning, two in the evening, flossing after dinner, rinsing without sugar. For her braces she used an interdental brush and wax so nothing would hurt. She used to be ashamed of her smile; now she wore it like a badge. She had an app on her phone that played waves as she brushed her tongue. Her grandmother used to say that healthy teeth were like a door to courage. This week, a mobile dental surgery with a blue logo pulled up to their school. Lena set up a Smile Guard circle with Olek and Maja from parallel. They made posters about the two-minute rule, flossing once a day and plaque. The dentist on the bus said that water after sweets is a small shield. Then someone slipped them a card: Tile grows under the school, check the sinks. The card smelled of mint and caramel, like the toothpaste no one buys. After lessons they went down to the old changing room, next to the makeshift office in the basement. The door was closed, but the mirror above the sink trembled like a sheet of tea. They listened to the hum of the pipes, counting to twenty, as they always did when brushing. Olek took out the pH strips and Maja poured water into a cup. The water tasted suspiciously sweet, as if someone had added syrup to the pipes. Lena ran her finger over the tap; a sticky residue remained, like fresh tile. In the torchlight they saw a row of tiles arranged like molars. Something hinged between them, resembling a toothbrush handle. Lena applied her sonic brush, switched on the gentle mode and touched the handle. Vibrations went through the wall and the mirror opened like a model's jaw. The coolness of mint wafted from within, but there was also the smell of fudge. Maja tied the handrail with dental floss, as if it were a lifeline. Olek lifted the dental mirror, and something squeaked hard in the tunnel. Droplets glistened on the walls, like fluoride-flecked mouthwash. Lena squeezed the toothbrush like a torch, sensing that this was no ordinary mischief. A shape blurred from the depths, glistening with caramel, and then they heard a whisper: "Lena..."