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acid American animals appear atomic average base become birds blood body Book building calendar called cats cause cells centimeters changes chemical color common complete containing covered designed developed Dictionary disease distance Earth effect eggs electric elements energy engine example feet five four garden glass Group grow Health heat hour human inches insects invented kilograms kilometers known largest lead leaves length less light living machine mammals material mean measured metals meters miles million months moon move natural North occurs organisms original period person physical plants pounds Press produce Published range result scale Science side soil solar space species speed star structure surface temperature term tree United University usually Washington weight wood World York