Instr function seems to be vague

[movenk]

I issued the following commands and get answers which are not consistent ,Can anyone explain to me how the positions are generated?Thank s
SQL> select instr('Eagritt' , 'e') from dual;

INSTR('EAGRITT' ,'E')
--------------------
0

SQL> select instr('Eagritt' , 'a') from dual;

INSTR('EAGRITT' ,'A')
--------------------
2

[Rabbit]

Are you using lower case or upper case? It's case sensitive.