Every Human Being currently alive on this Earth will hear it, somehow, at the same time.
You may say ONE sentence, following American English grammatical rules as what classifies as a sentence.
What would you say?
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"Attention, people of Earth!" They'll be expecting me to continue, unaware of the fact that I just wasted my sentence. They'll be baffled for weeks, never knowing if I'll continue or not. Or, "Please, everybody raise your hands up and lend me your energy so that I can defeat Majin Buu!" [Edited on 12.06.2012 7:10 PM PST]