Playing with Words: Dialogue

I've always been a description sort of guy.  It's something incredibly important to writing--every story needs it.  I love to write description and scenery.  It's something that I've always found to be easy, that a lot of people find to be hard.

Dialogue, however, is an entirely different story.  Like description, there are very few stories out there that have no dialogue (there are some, yes, but they are few and far between).  Without dialogue, you're stuck with one character doing and observing things, and that's not very interesting.

I was going to write a dialogue exercise today.  I really was.  But, the more I thought about it, the more I realized that I don't actually have a good one to improve dialogue.  So, instead, I'm calling upon you: What is your favorite dialogue exercise?