The most precise timekeeping device of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I (15251504 BC). They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water. The Ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages. In the 11th century, Chinese inventors and engineers invented the first mechanical clocks driven by an escapement mechanism.
When were the first mechanical clocks created?
the 11th century

Synnecrosis is a rare type of symbiosis in which the interaction between species is detrimental to both organisms involved. It is a short-lived condition, as the interaction eventually causes death. Because of this, evolution selects against synnecrosis and it is uncommon in nature. An example of this is the relationship between some species of bees and victims of the bee sting. Species of bees who die after stinging their prey inflict pain on themselves (albeit to protect the hive) as well as on the victim. This term is rarely used.
What is the ultimate result of synnecrosis?
death

In the 20th century, Russian was mandatorily taught in the schools of the members of the old Warsaw Pact and in other countries that used to be satellites of the USSR. In particular, these countries include Poland, Bulgaria, the Czech Republic, Slovakia, Hungary, Albania, former East Germany and Cuba. However, younger generations are usually not fluent in it, because Russian is no longer mandatory in the school system. According to the Eurobarometer 2005 survey, though, fluency in Russian remains fairly high (2040%) in some countries, in particular those where the people speak a Slavic language and thereby have an edge in learning Russian (namely, Poland, Czech Republic, Slovakia, and Bulgaria).
Where was Russian schooling mandatory in the 20th century?
Poland, Bulgaria, the Czech Republic, Slovakia, Hungary, Albania, former East Germany and Cuba