Editing Tip of the Day: It's Dialogue Time, Bob!

Today we're bringing you another tip on how to improve your dialogue writing skills.
Remember who you're talking to, and what they know.
Have you ever read a passage of dialogue that started with something like, "As you know, Bob, first we had to disentangle the fragmabobulator from the zorgatron..." and then the dialogue goes on, listing all the info that person A (let's call him Fred) has about the situation.
Do you see the problem, though?

Why is Fred telling this to Bob, if Bob already knows it?

Obviously, Fred is telling Bob so that the reader can find out. But it's usually pretty boring and transparent. It's too much of an info dump and it tends to turn into a monologue.
There's a slightly better version, the "Do you remember the time we did X?" "Oh, yeah, and then Y happened?" This can work, if it's done well. But it needs to come up naturally; something should spark the memory organically.
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I remember seeing one famous author pull this off with a scientist character talking possibilities over with another character; when she realizes he has no idea how the science works, she stops to explain. In that case, it worked well because the worldbuilding had already established the second character would not have had access to scientific education, but was intelligent enough to understand. And they had a good enough rapport that the dialogue was interesting. It felt genuine, like two friends, not like a lesson.

Think about how dialogue progresses between two people who know each other in real life situations the next time you want to use dialogue to drop some important info on your readers. Dialogue is a great tool, but you have to take into account what the characters already know, and what they need to know, for the conversation to proceed, and not just what you want your readers to learn.