Don't hesitate to assign a group leader to give all idea contributors motivation and make sure everyone is having fun. When someone suggests something, they encourage him or her to speak up so others can share their thoughts.
Brainstorming, which requires a free-flowing, creative process for ideas, has its own rules that should be followed to achieve success. The most important of these rules is that people must avoid evaluating ideas during the process: quality and focus come at a later time.
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