In "Schoolhouse Mystery" by Gertrude Chandler Warner, the main characters discover the value of an old valentine. The statement highlights the significance of sentimental items, as even a single piece from the past can hold substantial worth, both financially and emotionally. This revelation prompts a deeper appreciation for seemingly mundane artifacts that can connect people to their history.

The story emphasizes the importance of curiosity and exploration. The characters' investigation into the valentine leads them to uncover more about their surroundings and each other. Through this adventure, they learn that mysteries often lie in overlooked places, inviting readers to recognize the potential value hidden within everyday objects.