action america american americans amount animal animals black blood business cause cells change changes chemical children circle class color control country definition describes determine difference earth energy equal equation expression family force function government heart house human identify increase information learning level light little living major match matter money movement natural north object ocean outcome oxygen people period person physical plant plants power president pressure process product provide question questions rights river school select sentence shuffled_answers social solve south space specific spelling statement story structure students study support surface system temperature times total trade united value volume water white world