Halloween had finally arrived and the leaves were dancing on the pavement. Lena had an astronaut costume and a shiny new torch. Olek was a fox, with paper ribbons rustling in his tail. The rain had stopped and the air smelled of sweet pumpkin and wet grass. The moon hung over the roof, thin as a claw of night. The parents stayed at the gate and waved them through with a smile. They had one house left, at the end of a quiet street. On the porch stood a lone pumpkin with a crooked smile. The house had its lights out, only the porch glowed with dampness. Instead of a candle, a green light was glowing in it. A string swayed on the railing, as if waiting for the wind. Someone had taped a note: "Don't knock. Listen. The key is waiting." "I think it's a joke," whispered Olek, but his eyes sparkled. Lena brought the torch over and looked through the cut-out eye. Inside lay a small tin key and a folded map. The map showed a playground and an old oak tree with a hollow. Someone had also drawn a swing and a dot with the word START. On the edge was the phrase: "Insert when the moon is thin." "Thin as today," Lena remarked and bit her lip. So they ran to the oak tree behind the school, along the dark path. The tree hummed quietly and the hollow was like an open mouth. Under Lena's fingers the bark had a key-shaped spot. She slid the key in, Olek held the torch, and the night knelt down. The cool air blew in their faces, smelling of earth. Lena glanced at Olek; they both nodded at once. Something clattered inside, and the oak sighed as if it were alive. A whisper flowed from the depths that knew their names. Then a distinct knocking sounded three times from inside.