Short answer? Don't.
But we all have situations where we have to say that something isn't possible or we need more time or someone else would be better at that job. So what's the best way to communicate that to a boss?
I started thinking about this question this morning when I received an email from my undergraduate assistant. On Monday, I asked her to complete a small project for me, something which would take me about 2 hours to complete, so I'm guess it will take her 3-5 hours. Knowing that she is a student and has other responsibilities, I requested that the project be completed by the end of the day on Friday.
Her response to the message went something like this:
I'm swamped with midterms this week so the soonest I'll be able to work on this is Friday morning. And I don't know what Boss 2 is going to have for me, so I can't guarantee I can get this done Friday.
Maybe it was the fact that I'd spent my weekend grading 160 papers starting both Saturday and Sunday at 9am and ending around 3am, maybe it's the fact that I currently have two full time jobs and run a major committee and sit on the board of a non-profit, maybe it's that I don't remember being busy as an undergrad because all I remember is having fun, but this response irked me. Seriously, you can't come up with maybe 1 hr every day to do work that you are being paid to do?
And maybe it's the fact that I teach communication. But here's what I would have written:
Thanks for emailing me this project so I didn't have to stop by your office. I will try to get this done by Friday evening, but, like you, I have a lot going on this week. Would it still work for you if I had to send it Saturday morning?
The difference between these two messages has to do with a focus on the audience. The first one is all about "I," what's going on in MY life, what MY schedule is, that I feel overwhelmed by having TWO bosses. The second one is having a conversation with the reader and trying to manage expectations. It shows appreciation for one small part of the project and indicates an intention to meet the deadline. But it also asks permission to shift the deadline slightly.
Instead of telling the boss that she isn't going to get the work done, the second message asks if it's ok to adjust the deadline if it comes to that. And frankly, if you aren't going to meet a deadline, your boss would rather know it up front, but then you should clearly indicate when you do think you can get it done by.
Bosses know their employees have lives; they also know that they are paying employees to do work. So messages telling a boss what we can't do should really be focused on what we can do. All professional messages should focus on a positive potential future, a way of resolving issues rather than dwelling on them. And ask questions. What's the worst that could happen?
Short answer? Don't.