When the tree falls it causes pressure waves through the air. This happens whether someone is there to hear it or not. If there is someone to hear it, those pressure waves trigger neural activity in the brain of the listener and the listener experiences a sound. The tree falling is the trigger, the experience of sound occurs within the brain/mind of the listener.

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The reason why sky looks blue during the day is because when the sun rays hit the atmosphere they get Kansas City Chiefs NFL Grinch Candy Cane Personalized Xmas Gifts Christmas Tree Decorations Ornament_78228050 1 their constituent colors and it’s the blue color that scatters the most so we see that the sky is largely blue. But during the sunrise and sunset we see that the sky near the sun gets red and orange it’s because now the sunlight has to cross through more distance in the atmosphere and hence now we see more of red color than blue.

That depends on the context – Kansas City Chiefs NFL Grinch Candy Cane Personalized Xmas Gifts Christmas Tree Decorations Ornament_78228050 1” is correct if you are talking about, say, the native population of a country and don’t want to repeat yourself by referring to them as “the aboriginals” or “the saami” in every sentence. There are some other uses, but you would mainly use it for a cultural subset like that.