Here's why:
* Interrogative sentences ask questions. They usually start with question words like "who," "what," "where," "when," "why," or "how," or they end with a question mark.
* "Thought" is a single word. It doesn't form a complete sentence with a subject and verb, and it doesn't convey a question.
To make it an interrogative sentence, you would need to add more words and structure, like:
* "What did you think?"
* "Do you think it's a good idea?"