The sun was just hiding behind the red roofs of Primary School No. 7, tinting the corridor with a warm orange streak. It smelled of plasticine, soap and something else-mint. Hania stood on tiptoe to attach a poster of smiling paste to the door of the dentist's surgery. The poster said: "Two minutes, circular movements, a smile for days". - Higher-up, Olek replied, holding up a roll of tape. - So that everyone could see right away. - I can't go any higher," gasped Hania, but she raised the poster a few centimetres anyway. Hania was eleven years old, serious-eyed and had a notebook full of stickers with teeth in lace caps (because why not). Olek, a year younger, wore a peach bucket hat and thought brushing was like a sport: first a warm-up, then a series of moves and a rinse finish. In the office, which the pupils called the Smile Cabinet, the tiles with the fish shone, pictures of brushes hung on the walls as if from an art catalogue, and in the middle stood the pride of the club-a two-minute hourglass. Not just any hourglass: as tall as a thermos, inside the fine blue sand shimmered like a beach under the stars. - 'If all four Bs pass the two-minute brushing test tomorrow, you'll get a gold sticker for the board,' smiled Dr Rose Lśniewska, the school dentist, whose real name was that her teeth shone even in the rain. - And tonight... Night Brushing! Have you heard? Lectures, competitions, no sugar, but with raspberry juice. - And with XXL dental floss - Olek winked, as he had just developed a sample of floss and compared it to serpentine. - OK, you brave helpers, I will jump to the storeroom for more cups and pastes. Keep an eye on the office - said Dr. Róża and disappeared into the corridor. After a minute of silence, Hania noticed something strange. The sand in the hourglass had stopped flowing. It stopped halfway, as if someone had pressed pause. One grain hovered at the neck, embarrassed that it didn't know where to fall. - Can these hourglasses do that? - Olek put his eye to the glass. - Hello, time, we're working! A low sound came from the sinks, something like the purring of a kettle that can't decide whether to boil anymore. The mirror on the wall fogged up and, although no one breathed in its direction, words blossomed on the steam: Scrub...scrub.... - This... someone is making a joke? - Hania swallowed her saliva and immediately thought it was a good sign: saliva takes care of teeth. - Mr Felix? On the floor, under the cupboard, tiny, shiny crumbs were scattered. They didn't look like sugar. More like salt crystals, but they had a slightly minty smell and, when Hania touched her finger, they lit up like fireflies. - Don't bite it! - Olek jumped up and down. - How about... er... cleaning powder? - Powder doesn't blink - replied Hania. Her finger glittered as if she had dipped it in stardust. - Look. A murmur came again from the pipes under the sinks. This time like a breath: in and out, cool and minty. The hourglass jumped a millimetre, as if someone had nudged it from underneath. - 'Okay. Either the pipes have taken offence or someone wants to tell us something - Olek looked around to see if Dr Rose was around. - In the corridor I heard Mr Felix telling us that there are still pipes from the old swimming pool under the school, in the old part. Connected like a spider web. - The swimming pool hasn't worked anymore for years,' Hania recalled, but a glance into a dark corner of her locker made her feel shivers. There was something about the mysteriousness of the pipes that tickled the imagination. - Why don't we check it out tonight, during the Night Brush? Together, and not alone, because they'll all be there. The evening came faster than they could eat dinner. The gymnasium was transformed into a festival of smiles: a stand with funny brushes with glitter, a water stand with a tap that tasted like strawberries (it's just plain water, of course, but the message said to "taste it like strawberries"), a nurse's table with models of teeth so big you could brush them with a broom. Dr Róża Lśniewska spoke cheerfully: - Two minutes in the morning, two minutes in the evening. Circular movements, gently against the gums. If you have braces, don't forget to floss. And remember: Water is the friend of your mouth! Applause drowned out everything for a moment, but Hania and Olek communicated with a look. The hourglass, now set up on the podium, shone more than usual. As Dr Rose turned to hand the microphone to the director, sand... began to flow upwards. The particles climbed like salmon in a river, and the whole crowd did an "ooo". - Special effect! - exclaimed someone. - Super! - It's not an effect... - whispered Hania. She felt it in her skin, like a cool blast: a plume of mint and sage-scented air flowed from the direction of the dentist's office. At the same time, one of the washbasins in the room next door went noisy and released bubbles of foam, as if someone was brushing their teeth under water. - 'Let's go,' decided Olek, grabbing his cup from his rucksack and his own green-striped brush. Hania took her purple brush and a small packet of floss that Dr Rose had given her on an earlier visit 'just in case'. The corridor was semi-dark, the lamps buzzed and fluorescent sticker arrows guided them as a path to the surgery. The door stood ajar, though Hania remembered closing it. Inside again there was something like breathing, only clearer, with a rhythm. Shuru-shuru. Shuru-shuru. - 'It's just the pipes, Hania,' said Olek as one would speak to a younger sister, although he was the younger one. - Pipes have a rhythm too. A thin trail of mint-coloured crystals lay on the floor, leading to the second door, the one almost nobody used. They led into the old part of the school-the former swimming pool. Mr Felix kept buckets, old blackboards and big plastic exercise teeth there. The door handle was cool and smooth. Hania felt her fingers shake slightly-with emotion, or maybe from the chill that slid out from under the door. - On three,' she said. - One... two... The door gave way without a third. Inside, it smelled damp and minty. The former swimming pool was now a dry, wide pit clad in new tiles. At the bottom were props for preventative classes: a giant plastic jaw, a roller pretending to be a tongue, a brush as big as a paddle and a stand with a roll of dental floss so huge it could wrap around a gymnasium like a balloon. Something was wrong, however. The floss itself began to unroll. First a few centimetres, then a metre, five, ten, until the shimmering white ribbon flowed over the tiles like a river and slipped into the steel drain grate at the bottom of the pool. At the same time, in the corner, behind the large artificial jaw, a fine cloud of foam rose and... fell, leaving clear, tiny footprints as if from... well, from someone's footsteps. - Can you see it? - Olek crouched down, holding the end of the thread in his fingers. - She is pulling. It feels like something is pulling on it. - 'Don't pull with all your might,' warned Hania automatically, recalling Dr Rose's demonstration on how to handle the thread gently, 'like a spider's web, but firmly. - Rest your wrist, small movements. Do you hear? - She added after a while. - That sound? There was a clatter from the grille, as if someone was unclogging the metal cover from underneath. One, two, three. Then again: shoo-shoo. The hourglass standing on the shore-which, strangely enough, was here by itself, even though no one had moved it- shuddered and turned around as if on an invisible hand. The blue sand flowed, but not downwards, but horizontally, forming a thin, glowing streak that hovered over the grating like the hand of a clock showing straight-in. - Is this... an invitation? - whispered Olek. - Or a sign? Hania tightened her fingers on the brush. There was still a bit of paste stuck in her hair. She wanted to laugh at the absurdity of it-the night, the pool, the thread in the pipe, the hourglass pretending to be a compass-but the goosebumps on her arms told her to keep quiet. She took a step forward and leaned over the grate. A chill and... freshness came from below, as perfect as if someone had managed to seal the first day after a downpour in a bottle. The sound became clearer. It no longer sounded merely like a breath. It was quiet, but it folded into words: - Two... minutes... here we go.... Olek looked at Hania. Hania looked at Olek. The thread tightened suddenly more, like a tow rope. The trembling hourglass moved a centimetre, then another. The lid of the grille lifted a hair-literally the thickness of a hair-and someone, or something, from inside knocked three times. One. Two. Three. - On the count of three we lift," said a voice from under the grille, this time quite clearly, as if speaking through a pipe straight into their ears. - Ready?