At the end of the garden, right next to an old fence, there was a large oak tree. Its branches were like arms that hugged the sky. On one of the branches, Dad had built a little house. It had a window with a curtain of green leaves, a small terrace and a rope ladder. Lena and Olek loved to go up there. That evening they took a starry blanket, a moon-shaped lantern and a thermos of raspberry tea. Pikuś, their little white dog with a patch on his eye, climbed up behind them, slowly, step by step, clawing at the rungs with his claws. Mum waved from the kitchen window. - When it gets very dark, call out, okay? - she called out warmly. Lena nodded, and Olek pressed the soft blanket to his chest. He could already smell the leaves in the air and a hint of adventure. The late afternoon tinged the clouds pink. The wind moved the leaves of the oak tree: shhh.... shhh... The distant buzzing of the city and the close chirping of sparrows could be heard. The cottage became cosy. The children spread out a blanket, set up a lantern and hung paper stars on nails. - Look, Pikuś, this will be our evening on high - smiled Lena. Pikuś wagged his tail and stuck his nose under the blanket, as if he wanted to find a hidden treat. Olek poured tea into two small cups and placed three crispy biscuits next to them. For a while they just sat, sipping the warm drink. - 'Did you know that Grandpa gave me a key today? - Lena said. She took a small silver key out of her pocket. It had a star on the end. - He said: "Just in case." He just didn't say what it was for. Olek opened his eyes wide. - Maybe for the treasury? Or into the candy stash? - he laughed, but immediately became more serious, because Pikuś started tapping his nose on the floor again. The dog turned his ears up and barked quietly, as if on tiptoe: wuf. - What have you got there, Pixie? - Lena moved the lantern lower. The glow of the moonlight glided across the boards. Olek knelt down and outlined a darker spot on the wood with his finger. It was a knot, but a strange one, because it was shaped like an acorn. - I'll tap," whispered Olek, a little bravely and a little cautiously. He tapped his finger: tap. It responded from below: tap.... like an echo, only softer. The children looked at each other. The wind quieted, as if it wanted to listen to itself. Lena leaned even closer. - Can you hear? - She asked. Olek nodded his head. Pikuś lowered his muzzle to the gap between the boards and wagged his nose. The boards were a little different here: as if they were thinner, as if someone had once arranged them to fit together like a jigsaw puzzle. - I think this is... I think it's a door," Lena whispered. She blew the pollen off the wood and brushed away the tiny leaves that drifted from the terrace. Under her fingers she felt cold metal - a small round circle, flat, hooked into the board. - The wheel! - Olek was pleased. - It's like a real flap! The lantern flicked on and a dash of pale light seeped through a thin crack in the floor. It wasn't yellow like the torchlight. It was bluish, like the light of ice or a winter morning. A sound came from below, as if someone had quietly rung a tiny bell: ding. Pikuś withdrew his paws and sat down, tilting his head. His tail stopped wagging. Olek hugged him, felt his fur warm. And the air that blew from under the boards was as cool as a morning on the lake and smelled a little of rain, a little of resin. - I think that's the key calling us," said Olek in a whisper. Lena looked at the star on the key. It fitted the moment the way a snowflake fits winter. Her heart beat faster, but not from fear, but from a curiosity that tickled in her stomach. - Just quietly," she muttered. Sliding her finger into the metal circle, she felt it smooth and cold. With her other hand she held the key as if it were a good talisman. Olek held the lantern to keep the light from escaping, and counted Pikus' breaths: one.... two... three... Suddenly, a jay perched on a branch above the house and shouted: kra! Everyone jumped up. The lantern rattled against a board and rang thinly with glass. The blue dash under the floor became a little wider, as if it had also moved. - Did you hear that? - Lena asked. Olek nodded so slowly that his hair danced just a little. Pikuś barked quietly again - wuf - but did not take a step forward. Instead, his ears were like two small sails, straining against the wind. Lena touched the metal wheel and tried to lift it. Something inside shook slightly: a click. The boards trembled like strings. The bell rang a second time: ding.... and fell silent, waiting. - Maybe we should try the key first? - Olek suggested. Lena groped around the edge of the board. And indeed, right next to the wheel she found a tiny hole, one that the star on the end of the key fit into. Another gust of cold snagged their ankles. - 'I think Grandpa knew,' muttered Lena, not sure herself whether she was talking to Olek or Pikus or the oak tree. She slid the key carefully into the hole. It fit like a glove. The blue light trembled, as if dancing. The evening air rustled over the garden. A single yellow leaf danced on the terrace of the cottage. The shadows inside were not scary - they were soft and full of whispers. The children huddled together, but not to hide, just to be even closer to what was about to happen. - 'I'm ready,' whispered Lena. - Are you? - Together - answered Olek, gripping the lantern more tightly. Lena tightened her fingers on the wheel and touched the key with her other hand. - One... two... - she began counting silently. She didn't have time to say "three" because there was a sudden, clear knock-knock-knock...