On Monday a dentobus arrived at our school, white as chalk. I was fourteen years old, braces glistened on my teeth and my gums still rebelled. The hygienist showed the plaque under the UV lamp, it glowed venomously. After a staining tablet, I saw a stain on a molar, strangely resembling a key. - Scrub twice a day, floss, rinse, don't snack between meals, she reminded me. I nodded, though I was already doing it, sometimes more diligently than I do my maths homework. At a stand near the dentobus, they were handing out new sonic toothbrushes for those who wanted them. Mine vibrated as if it spoke to me mysteriously in Morse code. The bristles hid a miniature capsule that fell straight into a cup of liquid. I took out the paper, unrolled it carefully, helping myself to the taut floss. The inscription proclaimed, Stone grows; free the key from under the enamel before it breaks. Tartar - like tartar, the kind that the scaler on a visit scrapes away with a buzz. I thought of James's prank, the school prankster, and tucked the capsule into the case. After class, I went back to the bathroom because the stain wouldn't let up. I chewed sugar-free gum, but in my mind I was counting visits to the dentist and possible tooth decay. Liquid dots appeared on the mirror, arranged in an arrow. The arrow led to an old dentist's office, padlocked for years. I thought of a locker and a key, like from detective puzzles. I took with me a toothbrush, floss, a handheld irrigator and a bottle of mouthwash. I threaded the floss under the baked-on padlock, as if between my teeth under the gum line. The lock clicked all too easily, as if waiting for someone to actually floss. Inside, it smelled of mint and metal, and the scaler buzzed somewhere in the shadows. The UV light came on by itself, revealing the marks on the floor like a glowing tile. The marks formed the outline of a tooth, and the same key shape flashed in the centre. I tensed my cheeks to push back my lip and touched the molar with the end of the toothbrush. The toothbrush whined and pointed to the cabinet next to the steriliser, like a soft-bristled compass. The cupboard door shook, someone on the other side grunted through a mask. The light suddenly went out and a gloved hand clamped down on my wrist.