Custom prompt templates
The stuff chain type in Langchain uses a specific prompt with question and context documents incorporated. This is what the default prompt looks like:

Use the following pieces of context to answer the question at the end. If you don't know the answer, just say that you don't know, don't try to make up an answer.
{context}
Question: {question}
Helpful Answer:
We can, however, provide our prompt template and change the behavior of the OpenAI LLM, while still using the stuff chain type. It is important to keep {context} and {question} as placeholders.
Experimenting with custom prompts
We can try using a different prompt template, so the model:

Responds with a single-sentence answer if it knows it.
Suggests a random song title if it doesn't know the answer to our question.