What makes the bid-ask spread widen or tighten?
The spread is the best ask minus the best bid.
What events widen the spread? What tightens it? Can the spread change while the mid stays put?
Show a hint
The spread depends on both best quotes. Think about every way a best quote can move outward (wider) or a new order can appear inside (tighter).
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.