Your friend is visiting from out of town. You're getting ready to go out to eat together, and you're deciding where to go. Your friend asked if you knew of a good hamburger restaurant. But you don't eat hamburgers often, so you don't really know of anywhere good. You say:
You know, nothing really comes to mind.
"You know" is a phrase that introduces a sentence. It has lots of different uses, but one use is to make it sound softer when you have to disappoint someone or say "no" to their request:
In the example above, you don't have any good recommendations for hamburger restaurants. You have to disappoint your friend, so you start with "You know" to soften it.
This expression means "I can't think of anything." Use it when you're trying to remember something or come up with ideas but can't.
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