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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 twin is one of two offspring produced in the same pregnancy. Twins can either be identical (in scientific usage, "monozygotic"), meaning that they develop from one zygote that splits and forms two embryos, or fraternal ("dizygotic") because they develop from two separate eggs that are fertilized by two separate sperm. In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple...

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting. All known liquids, except liquid helium, freeze when the temperature is lowered enough. Liquid helium remains liquid at atmospheric pressure even at absolute zero, and can be solidified only under pressure...

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...