Earthisanoceanplanet.Aboutthreebilionyearsago,lifeonourplanetbeganinthedeepblueseas.Ninety-ninepercentofthelivingspaceonearthisintheoceans.Sowaterisveryimportantforus.Thewaterisbeingwidelyusedforelectricity

　　agriculture,homeuse,industry,transport,entertain---ment,andsoon.Butthemajorityofpeoplehavenotrealizedthisproblem.SincechangesinsalinityandtemperatureaffectWater'sdensity,thewaterintheoceanisalwaysmoving.Densewatersinksandlessdensewaterispushedtothesurface,Thiscirculationaddsenergytothemarineecosystemsandmovesnutrientsaround.Isn'titamazingthatasinglesubstancecanbesoimportanttooutplanetandeventhewholeuniverse?Water,whichseemssosimpleandcommon,iswhatmakeslifepossible.Ofalltheresourcesonearth--oils,gas,goldandsoon--nothingisaspreciousasadropofrain.Learningaboutthepropertiesofwaterhelpsusunderstandlifeonourplanet.Themostimportantthingwecanlearnaboutwater,however,isthatprotectitanduseitwisely.Ourfuturedependsonit.

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