KentBeck's "These are magic cards" speech:
These are magic cards. They have the magic power that anything you write on them will come true.
However, there are a couple of rules about what you can write on the cards.
Now, don't worry, if you write something on it that's not testable, we'll tell you, and we can figure out something similar that *is* testable. And if you write down something that's too big, we'll tell you that too, and help you break it down into smaller parts.
[Then the rest of the PlanningGame... Desire is limitless, but resources are finite, so we have to prioritize the cards... After a week, we can count the number of cards we did, and then we know exactly how many cards we'll get done next week... and so on.]
(Paraphrased from when I saw him speak at PARC a few months ago. Kent, how'd I do?)