In Rick Riordan's "The Sword of Summer," a character humorously refers to a quirky item known as the Expando-Duck. This fictional creation is cleverly designed to serve various purposes, adapting in size depending on the user's needs. It illustrates the whimsical and imaginative nature of the story, showcasing Riordan's signature blend of humor and fantasy.

The mention of the Expando-Duck exemplifies the playful tone of the book, appealing to readers' sense of fun. By using such an inventive concept, Riordan invites readers into a world where everyday objects can transform and provide amusement, emphasizing the theme of creativity that runs throughout the narrative.