Newbie: Why does this tiny function declaration fails?

Hi,
after writing my first script in PL/SQL, I tried to declare a function which failed. I tried the "new_add_month" example from the book "Oracle PL/SQL" which faild. After that I wrote this tiny program to reduce complexity:

FUNCTION banana (tomato IN NUMBER)

        RETURN NUMBER
   IS

       potato NUMBER;
   BEGIN
      potato:=tomato+1;
    RETURN potato;
    END banana;

The german reply said, that potato is not declared. I'm working as user 'internal' as SYSDBA.

Now the great question: Why does this declaration fails???

  Thanks for your help

                  Ralf