illegal expressions

       Person p1 = new Student();
       Person p2 = new PhDStudent();
//       PhDStudent phd1 = new Student();
       PhDStudent phd1 = new PhDStudent();
//       Teacher t1 = new Person();
       Teacher t1 = new Teacher();
       Student s1 = new PhDStudent();

//        s1 = p1;
//        s1 = p2;
        p1 = s1;
//        t1 = s1;
        s1 = phd1;
//        phd1 = s1;