There was a cosy group gathered about the open fire in the Birdseys' big, comfortable, and not too tidy living-room. At a large center table, drawn close to the blaze, sat Carol and Sue, scribbling away for dear life in two large, fat note-books and covering the table with many trial sheets of mathematical figuring. They were exact opposites in appearance. Sue was tall and slim to the point of angularity. She had dark eyes, and her dark hair was coiled heavily about her head. Carol was short and plump, with dreamy blue eyes and wavy auburn hair that still hung in a thick braid. On the davenport, curled up like a kitten in one corner, sat the Imp, or "Bobs," as she was generally called,--her chin propped in her hands, a book balanced against her knees. In sharp contrast to the other two girls was her tiny body and dark, straight hair, and the big blue eyes that could at one moment gaze with liquid, angelic candor, and at the next snap with impish mischief. There was mischief in them at the moment, as she stared reflectively at the two girls bent over the table. Unaware of her gaze, they scribbled on, comparing notes at intervals. "Do you get the answer 4ab(ab+2bm) ½ to your third problem?" presently inquired Sue, without looking up. "No, I don't!" moaned Carol in depressed tones, pushing aside her work and running her fingers through her hair. "I don't get anything at all." "Well, don't worry. Let's see what's wrong. Hand over your work and I'll compare it with mine," said her companion soothingly. She dragged Carol's note-book toward her and compared it with her own. "Oh, I see what you've done!" she exclaimed in a moment. "In the first equation you didn't put down--" At this instant the Imp, whose eyes had been smoldering with suppressed mischief, yawned loudly, stretched herself, and remarked with apparent irrelevance: "It's a long day when you don't go to school, isn't it?" Both girls sat up with a jerk and surveyed her sternly. "Do you mean to say that you haven't been to school to-day?" they demanded in a breath, and Sue added, "I'd like to know why not." "I had a bad headache this morning, Susan," explained the Imp sweetly. "Mother let me stay home. I was all right by two o'clock, though. Louis and I had a game of basketball before it began to rain." Her companions glanced at each other with a meaning expression, none of which was lost on the Imp. With a grin of satisfaction, she proceeded: "His aunt called him in just before we finished, and he didn't come out again." With a visible effort, Sue inquired: "Did he say why he wasn't at school to-day? We thought it rather queer when he didn't come, but perhaps, after the strange thing that happened yesterday--" This was precisely the trap into which the Imp had planned that they should fall. "I didn't ask him," she remarked, with exasperating calm. "No doubt you didn't," retorted Carol heatedly, "but perhaps he told you without being asked."