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