Jeremy Donovan's study of TED suggests that a good way to create an impactful speech is to answer a question. Think of an important question that interests you and will interest your audience. Then answer the question in the format "(Action) so that (outcome)." For example, if the question is "How can you become more ecologically aware?" the answer could be "Use one less paper towel than you currently use so that you reduce your paper usage ."
Professional Speech Coach
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