About a question at a job interview.

[Why Child]

“When you set out to do something, how do you know when you’re done?”

I went to a job interview, and that was one of the questions I was asked. It confuses me. If you know what you are trying to do, don’t you know when you’ve done it?
I answered that once I have accomplished whatever it was I wanted to do, I know that I am done. He said, “Yes, but how do you know when you’re done?” I am sure my perplexity showed. (I thought, “Maybe he’s looking for problem-solving skills, and/or ability to prioritize. Ok, I’ll fluff it a little.”) I responded, “Depending on how great a task I was trying to achieve, I would work out the logical steps in order to get it done. I would finish them in the necessary order, and once I had reached my goal, I would know I was done.” Again, he asked, “Yes, but how do you know?” I asked for an example of a goal. His reply, “Well, anything, really.” I hated to say it, but I finally told him that I must not understand the question.
What was he looking for? My "real life" friends don't get it, either. Does it seem a really simple question? When you’re done, you’re done!