“Have you all gone crazy, or have I?” cried poor Chet, looking still more bewildered.  But suddenly Teddy saw light.

“You mean the musical ghost,” he cried, laughter in his voice.  “The one that has had us chasing down flights of stairs on dark nights?”

“With the chills running up and down our spines and our hair standing on end?” added Ferd, following his lead.

“The very same,” responded Mrs. Gilligan, the gleam deepening in her eyes.

“But how did you catch it?” asked Violet, for the girls, all except Billie, who had originated the idea, were as much in the dark as the boys.

“With a trap,” said Billie, her own eyes beginning to sparkle.

“But who thought of it?” Violet insisted, ignoring the sarcasm.

“You see before you the girl who invented it,” said Billie with a chuckle.

“Great pumpkins, another inventor!” groaned Ferd, and sent them off into a spasm of laughter.

“Oh, tell us about it, Billie,” Laura entreated.  “You can be the most aggravating thing!”

“Stop calling me names or I’ll never tell you,” threatened Billie, at which Laura looked as meek as Laura could ever look.

Thereupon Billie recounted to an interested audience the events that had led to her idea that it might be a rat that was making a joke of them all and how she had decided to put her idea to the test.

“Say, think of getting excited about a mouse!” cried Ferd incredulously, when she had finished.

“It wasn’t a mouse—­it was a rat,” corrected Billie.

“But it might have been a mouse,” Ferd protested, but Billie broke in again.

“No it mightn’t,” she said decidedly.  “A mouse could never have made noise enough for us to hear when we were upstairs in bed.”

“Right you are,” said Ferd, taking off an imaginary cap to Billie.  “I have to hand it to you, Billie—­you’re right there.”

“You said it that time, old man,” murmured Teddy very softly, but Billie heard him and looked up at him with laughing eyes.

“Come help us open our trunk,” she said, turning away suddenly.

“Whose trunk is it?”

“Where did you get it?”

“Looks as if it had come out of Noah’s ark.”

These and many more comments piled one on top of the other as the boys looked at the old trunk, which did indeed appear old enough to have satisfied the most ardent collector of antiques.

“Why, it’s my trunk,” said Billie, when she could make herself heard above the babble.  “We found it in the attic.  But I don’t see what difference it makes where we got it,” she added impatiently, getting down on her knees once more and shaking the trunk as if it were to blame.  “Won’t you please get busy and open it, boys?  Aren’t you a bit curious to see what’s inside?”

“Is there a key?” asked Ferd, and Billie looked up at him in despair.

“Of course not, silly,” she said.  “Don’t you suppose we’d have had it open ages ago if there had been a key?  You’ll have to break it open, or pick the lock, or something.”