The Princess and the Pea: The Queen and the Pea
When the wannabe princess showed up at the palace door to escape from a passing rain shower, the Queen offered her a room for the night. While the suspect princess was dining, the Queen went to personally arrange the guest chambers. She made sure to bring along a pea, and then ordered her servant to acquire 20 mattresses. Thus operation Detect-a-True-Princess was begun. The Queen knew her plan was foolproof. The servant wasn’t so sure, but mostly just dreaded having to drag 20 mattresses away again in the morning.