Final answer:
The vivid description of Mr. Bixby's smile and the laughter of the group contributes to the surprise element at the end of the story in 'hearts and hands'.
Step-by-step explanation:
The best detail from "hearts and hands" that contributes to the element of surprise at the end of the story is the vivid description of Mr. Bixby's "bland" smile contrasted with the uproarious laughter of the rest of the group. This detail adds to the element of surprise because it reveals that the group was tricking the narrator, making him feel meaner than ever. The tension starts to dissipate after this revelation, indicating the falling action of the story.