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Outer space (often simply called space) is the void that exists beyond any celestial body including the Earth. It is not completely empty (i.e. a perfect vacuum), but contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation, magnetic fields, and neutrinos. Theoretically, it also contains dark matter and dark energy...

China is seen variously as an ancient civilization extending over a large area in East Asia, a nation and/or a multinational entity. With nearly 4,000 years of continuous history, China is one of the world's oldest civilizations. Prior to the 19th century, it possessed one of the most advanced societies and economies in the world; but through successive dynasties it then missed the Industrial Revolution in Europe and began to decline...

A train is a connected series of vehicles for rail transport that move along a track (permanent way) to transport cargo or passengers from one place to another. The track usually consists of two rails, but might also be a monorail or maglev guideway. Propulsion for the train is provided by a separate locomotive, or from individual motors in self-propelled multiple units...

A POTD is an acronym for "Photo of the Day", "Picture of the Day", "Post of the Day" or "Poll of the Day". It is a term most commonly used by online digital photography communities as an honor to outstanding photos that are selected daily from among that community's membership. They can either be tagged as POTD or displayed in a special page on that site...