Luna and the Clockwork Door

Luna woke up at 3:07 in the morning to a sound like a heartbeat made of gears. Tick. Tock. Tick. On the wall beside her bookshelf, where there had only ever been a poster of the solar system, there was now a small round door with brass hinges.

Bolt: "Please don't scream," said a voice from her desk. It was Bolt, the homework-helper robot her grandmother had built. His eyes glowed a nervous blue. "I've been monitoring that door since 2:51. It is 37 degrees warm and it is ticking at exactly the speed of a sleeping cat's heart."

The door ticked a little faster, as if it knew she was awake.