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.
 
1. It has to be testable.
 
2. It has to be something that can be created in less than a week.
 
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?)