Ruby the Copycat is a story about a girl named Ruby who imitates the behaviors and words of others. The story introduces vocabulary words like "trudged" to describe walking slowly, "gushed" to describe talking excitedly, and "frustrated" to describe the feeling of not being able to do something after repeated tries. It also defines words like "coincidence," "modeled," "loyal," and "recited" that describe copying or repeating actions, words, or behaviors of others.