Why do you think Twain leaves Tom and Becky's whereabouts a mystery until the window reveals that Tom's gone missing? How does he then explain what happened?

Noticing that Becky is not at church, Mrs. Thatcher asks Mrs. Harper, "'Is my Becky going to sleep all day? I just expected she would be tired to death." Mrs. Harper says she had not scene Becky at all. The allows the readers and the characters to connect the dots to what happened.