Rob’s lips twitched.

“But, Betty—­” he began.  Then—­“Oh, do play fair,” he urged.  “You’re younger than I am.  Uncle Dick expects me to look after you.  Goodness knows I don’t want to pry into your affairs, but when you borrow fifteen dollars and then want twenty-five the same week, what’s a fellow to think?  If some one is borrowing from you, it’s time to call a halt; you’re not fair to yourself.”

Betty looked startled.  How could Bob possibly guess so near the truth?  She began to think that the better part of wisdom was to confide in this keen young man.

“Come on, Betty, tell me what you want it for, and you shall have twice twenty-five,” said Bob earnestly.  “I’ve most of my allowance in the school bank.  It’s all yours, if you’ll let me have an inkling of the reason you need money.”

“Well,” said Betty, slowly, “I didn’t promise I wouldn’t tell—­only that I wouldn’t tell Bobby or Mrs. Eustice.  It’s Libbie who has to have the money.”

She sketched Libbie’s story for him rapidly, Bob listening in silence.  At the end he asked a single question.

“Have you any of those notes asking for money?”

“Here’s one.”  Betty thrust her hand into the pocket of her sweater and pulled out the crumpled paper that Libbie had shaken out of the bottle that morning.

“Were they all written on this same kind of paper?” asked Bob, reading the note.

“Ye-s, that is, I think so,” hesitated Betty.  “I really haven’t noticed.  Why?”

“Because I don’t think any man wrote this,” announced Bob confidently.  “I think some girl at school has done it, either as a joke or to torment Libbie.”

“But it’s grown-up writing,” protested Betty.  “Though, come to think of it, we don’t know any of the girls’ handwriting,” she added thoughtfully.

“What girl would be likely to do it?” asked Bob.  “Can you recall a practical joker?  This is copy book paper torn from an ordinary theme book.  Yes, I’ll bet a cookie a girl wrote it.”

“Ada Nansen or Ruth Gladys Royal might do it to plague Libbie,” said Betty slowly.  “They don’t like any of our crowd, and Libbie is so good at French she turns Ada green with envy.  The more I think of it, the surer I am it is Ada.  Ruth doesn’t dislike any one actively enough to exert herself.”

“Ada Nansen?” repeated Bob.  “Isn’t she that girl we saw on the train and who plumped herself down in my seat?  I thought so—­I remember you told me.  Well, from the sidelight I have on her character, I believe she is the one at the bottom of this.  That will explain, too, why you never catch any one digging up the bottle—­she knows exactly when you are busy and when you are not.”

“Bottle!” said Betty explosively, to Bob’s amazement.  “Oh, Bob! this morning Miss Jessup was talking to us about association of ideas, and she asked Ada what bee meant to her.  We thought she’d say ‘honey,’ of course, but she said ‘bottle.’  Doesn’t that show—­”