Final answer:
The context provided is insufficient, but the idea of driving a Cadillac into Mississippi may invoke a serious mood due to historical racial tensions and contrasts of wealth and poverty, especially if it's during the Civil Rights era.
Step-by-step explanation:
Without the context of where the quote comes from, it is challenging to provide a definitive answer. However, the phrase “drive the Cadillac into Mississippi” may suggest several historical and cultural connotations that could cause a sudden halt in laughter. If this takes place during a time of racial tensions, particularly during the Civil Rights Movement, the idea of entering Mississippi might bring to mind the state's history of racial discrimination and violence, which would not be a laughing matter. Additionally, a Cadillac, being a luxury vehicle, entering a place known for its severe racial issues could represent a stark contrast of wealth and poverty, which might also diminish the humor of the situation.