I think I've found the second most exhausting thing in the world to do.
Which of course is to sit down and discuss functional requirements with a user. Users know what they want - they just have a hard time explaining it in a way that makes sense to programmers.
Oh well, the important thing is that the meeting was successful and the functional requirements for my current project at work are for the most part nailed down.
And for those of you who may be wondering, the most exhausting thing in the world is chasing a four year old through a park.