Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Engineering Challenge : Catapult

Engineering challenges are fun activities that are frequently assigned to students in physics or engineering classes, but they can easily be used outside of these venues. An engineering challenge usually has a simple goal and some rules about what sorts of materials you are allowed to use. These are great ways to get people thinking creatively and to foster teamwork and competition. Try an engineering challenge at a birthday party, office outing, scout meeting, or any other gathering where you want to have fun and exercise your creativity.

The objective in the Catapult challenge is to fling a tennis ball the furthest distance. Each team is given the same kit of supplies, which could contain things like wood, nails, rubber tubing, dowels, etc. Teams can construct traditional catapults, trebuchets or more creative flinging devices if they wish. In addition to the distance challenge, you might want to set up targets and give prizes for accuracy. Each team will be allowed to aim and make adjustments in an attempt to get closest to the target.
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For more mind exercises: http://www.braingle.com/mind/