Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Shape Hunting

Look around the room and try to find as many squares as you can. Picture frames, sticky notes and tissue boxes are some examples of objects that are square. Remember, rectangles don't count! This exercise helps you become more aware of your surroundings and helps improve your attention to detail. If squares are too easy for you, try circles or triangles.

You can do this exercise whenever you are sitting around doing nothing, whether it's on the bus or in a waiting room. If you have a friend with you, make it a challenge to see who can find the highest number of a certain shape.
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For more mind exercises: http://www.braingle.com/mind/