When you want to talk about a new topic, one way to introduce the topic is with "You know ___?" For example, you can use this phrase to start telling a story:

Hey, you know that book store on 12th Street? So I was in there the other day, and I was looking for a cookbook...

Or you can use it to ask a question:

You know that time when we got snowed in and didn't have to go to school for a whole week? What year was that?