An engineer explained to Steve Jobs why the Mac's boot time couldn't be reduced. Jobs cut him off and asked a different question: "If it would save a person's life, could you find a way to shave ten seconds?"
Then he went to the whiteboard. Five million people, ten extra seconds, every day. Multiply it out and the waiting adds up to 579 days of human time a year. "So if you make it boot ten seconds faster, you've saved a dozen lives."
A few weeks later Larry Kenyon came back having cut 28 seconds. Fifty-two down to twenty-four.
Nothing about the code had changed in between. What changed was the size of the problem he thought he was solving.
"Impossible" is usually just a verdict on a problem too small to be worth the effort.