Final answer:
Without the specific context of the story, it's not possible to accurately answer why Agatha gave Sophie a dead frog for her birthday.
Step-by-step explanation:
The question 'Why did Agatha give Sophie a dead frog for her birthday?' seems to be referencing material from a specific story or text that is not fully provided. Therefore, without specific context or knowing the characters and their motivations, it is not possible to accurately determine why Agatha gave Sophie a dead frog. The snippets provided discuss different characters (Sister Sara and Mrs. Tilley) and seem unrelated to the question posed. Without additional information, we can only speculate about themes or messages in the broader narrative such as surprise, humor, or an unconventional perspective on what constitutes a gift.