The old paper crinkled like autumn leaves under Charlie's fingers.
He found it tucked behind a loose brick in the basement wall. The paper was yellow with age and covered in faded brown lines. His sister, Rosa, leaned close. Her breath made little fog clouds in the cool air.
"It's our house!" Rosa whispered.
The drawing showed their living room and the kitchen. But there, between the bathroom and their bedroom, sat a door that didn't exist. Above it was a big red X.
"That room's not real," Charlie said. His voice trembled with excitement.
"Maybe it's not real *yet*," Rosa suggested.
They raced upstairs. In the hallway, they pressed their hands against the wall where the mystery door should be. Just wall. Plain, ordinary, boring wall.
"We have to find it," Charlie declared. "Someone put that X there for a reason."
"But how do you find a room that isn't there?" Rosa asked.
Charlie thought hard. "My science book says everything is made of tiny pieces called atoms. Maybe the room is hiding in the spaces between them!"
They tried their first experiment. Charlie knocked on the wall in a pattern: *tap-tap-TAP, tap-tap-TAP*.
