Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Negative Thinking

Negative thinking is usually a bad thing, especially if it occurs in the form of premature judgment [ For more on 'premature judgment' go here: http://www.braingle.com/mind/395.html ] of an idea. However, sometimes intentionally thinking negatively can help you find weak points in your solution.

Think about the worst parts of your idea. Try to identify all the defects that you can. Be as critical and superficial as possible and keep a list of all the negative aspects of your idea. Once you are done, go over your list and identify the key defects. See if you can improve them.

This can be especially helpful if you are going to present your idea to a group or person who is going to judge it. If you already know the top few things that are wrong with it, you can have some arguments or alternative solutions prepared in your defense.
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For more mind exercises: http://www.braingle.com/mind/