Mutating triggers / Mutating errors in triggers

create or replace trigger trig_mut
    before insert on emp10
    for each row
    begin
    insert into emp10(empno,sal) values (:new.empno,:new.sal);
    end;
    /
Trigger created.



SQL> insert into emp10 select * from emp;
insert into emp10 select * from emp
            *
ERROR at line 1:
ORA-04091: table SCOTT.EMP10 is mutating, trigger/function may not see it
ORA-06512: at "SCOTT.TRIG_MUT", line 2
ORA-04088: error during execution of trigger 'SCOTT.TRIG_MUT'