Some so-called UFO is just a bird, a plane, a balloon or even a plastic bag.
一些所谓的UF o只不过是一只鸟，一架飞机，一个气球，甚至只是一个塑料袋子。
Balloon: Large airtight bag filled with hot air or a lighter-than-air gas such as helium or hydrogen that can rise and float in the atmosphere.
气球：一种大型气密球囊，内充热空气或轻于空气的气体（如氦或氢），以产生浮力，可在大气中上升和飘浮。
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