How to use and what does this phrase mean,"Speaking of which".

"Speaking of which" is a phrase used to smoothly transition from one topic of conversation to another related topic. It typically implies that the topic being brought up is related or connected to the current discussion. It is often used when someone wants to introduce a new point or share additional information that is relevant to the ongoing conversation.

Here's an example to illustrate the usage of the phrase:

Person A: "I'm planning a trip to Paris next month."
Person B: "Oh, I've been there before. It's an amazing city with so much to see and do. Speaking of which, have you visited the Louvre Museum?"

In this example, Person B uses the phrase "Speaking of which" to smoothly transition the conversation from discussing the trip to Paris to bringing up an additional topic related to the trip, which is the Louvre Museum.

Overall, "Speaking of which" helps to maintain the flow of the conversation while also introducing a related topic for discussion.