what privileges needed for S2 for this function (Oracle 12c)?

I had this question after viewing grant user/role question.

Please read the above link for the 1st part to setup the test case.

Continue to 2nd part:
For simplicity, still use "sys" one more time
sqlplus login as sys

CREATE OR REPLACE FUNCTION S2.GET_ID
  RETURN number AS
  v_id   number;
BEGIN
  SELECT id
    INTO v_id
  FROM s1.t1;

  RETURN v_id;
END;
/

If compiling, the error showed
"PL/SQL: ORA-00942: table or view does not exist"
From 1st part, I have granted the privilege below:
grant select on s1.t1 to s2;

Questions:
1) why does it complain that it could not see the s1.t1
2) the final result is that I like to compile and execute "GET_ID" logging in as S2. What kind of privileges does S2 need? I am talking about the least privileges to execute the function as S2.

If any guru can shed some light on it, I will greatly appreciate it.