At the end of Chamomile Street stood a pond so smooth that it liked to pretend to be a mirror. And right on its shore grew an old willow tree. It hung its long branches all the way down to the water, as if stroking the waves. In summer it gave shade, in autumn it whispered about the leaves, and in winter it wrapped itself in snow like a scarf. Zosia and Tolek had known this willow forever. Zosia was seven years old and had her hair tied up in two springy ponytails. Tolek was six years old, with pockets full of treasures and shoes that always found puddles. Sometimes they were accompanied by a sparrow called Flea, as small as an ice-cream cone but as brave as a fireman's hat. One evening, just after the rain, the air smelled of wet grass and fresh rolls from the bakery. The pond twinkled with silver and the willow looked as if someone had sprinkled sparks on its leaves. Zosia put her ear to the trunk. - Can you hear it? - she whispered. Tolek put his ear to it too. Instead of the usual shur- shur, they heard three quiet notes, like on a tiny flute: ti-rii, ti-rii, ti-rii. The flea wagged its head and flapped its wings so fast that it made a little wind. Suddenly, a single leaf detached itself from a high branch. It did not simply fall. It circled in the air, whirled, and bright dots flashed on its green skin, arranged like stars. The leaf landed directly on Zosia's hand. It was warm, as if it were lying in the sun. - Look! - Tolek pointed to the trunk. - There... something is being drawn! Delicate lines appeared in the bark of the willow, first like a spider web, then more and more distinct. The outline of a small door emerged. It was shaped like an arch, and in the middle grew a round knob, similar to an acorn. Above the door shone three leaves arranged like notes on a stave. Zosia blew on the leaf in her hand. The shiny dots shifted and formed an arrow that pointed straight at the knob in the bark. The flea lowered its flight, beeped contentedly and sat on Tolk's shoulder. - Is this... an invitation? - asked Tolek very quietly. Zosia did not answer. She took a step forward and touched the knob. It was cool and smooth. She tried to twist it, but nothing happened. The knob was as still as a pebble. - 'Maybe you need a key,' muttered Tolek, pulling a pencil, a cap, a button and a piece of string from his pocket. None of the treasures matched. The flea sparrow suddenly lifted into the air and dived into the reeds. All that could be heard was a splash and a shush. After a while he returned, dragging something in his beak that looked like a feather, but was not a bird. It was silver, thin and trembled like a ray of water. - Where did it come from? - Zosia was surprised. The flea let go of the feather in her hand and swelled up proudly. As Zosia brought the feather closer to the outlined door, a silvery glow gently moved the lines in the bark. It was not a key to the hole. It was something like a brush for light. - 'Try drawing a twig,' prompted Tolek. Zosia stroked her feather over the bark, connecting the dots from the leaf with the notes above the door. Where she touched, the bark softened like a thick fabric and the trail lit up with a bright trail. All around it smelled of honey and rain, like when you open a window after a storm. - 'Oh dear,' said Tolek, making big eyes. - She is breathing. The willow really took in air. The leaves chimed quietly, as if someone had hung tiny bells on them. The pond grew quiet. Even the frogs stopped croaking. Lights slowly faded from the windows of nearby houses, and the sky above the water darkened into a garnet. Zosia and Tolek stood side by side. The flea sat between their shoulders to make it easier for him. Zosia squeezed the feather in her fingers, Tolek squeezed Zosia's hand. - Are you ready? - She asked. - Like a slice of jam - replied Tolek, nodding very seriously. Zosia drew the last line. The silver trace merged with the acorn knob and soaked into the bark. Something inside the tree tapped quietly: tick... yes... tick... yes... like the ticking of a very old watch. The door trembled and the knob moved a millimetre, then another. The leaves above them rustled faster. Tiny circles appeared on the water, as if someone unseen was taking small steps. Circles of light flashed in the bark, brighter and brighter, like ripples on a puddle when it rains. - Can you hear? - Zosia adjusted her ears. - There's someone... From the depths of the trunk came a sound they didn't know how to name: some singing, some ringing, some pages being turned. The knob twitched a third time. The flea slipped its head under the wing and then immediately put it out again, because curiosity was greater than slumber. The wind blew straight from the pond, bringing the scent of wet bark and sweet lime. Zosia felt her heart beat faster. Tolek swallowed his saliva and straightened up bravely. The knob moved a little more and... started to turn on its own. At the same moment, something whistled just off the ground, a single, very distinct: plums! came from the reeds, and a narrow, silver crack appeared on the bark, which lit up their faces like a torchlight. The door in the willow was just swinging open, and a shadow flashed on the other side, as if someone was reaching out for the handle from that side.