How to get a S-lock in a Transaction

[N!]

Hi:
    I tried to get a S-lock using following SQL:
   1. start transaction;
      SELECT * FROM test.t1 where id=1;
    But I found this way does not work. I changed it as following:
    2. start transaction;
        SELECT * FROM test.t1 where id=1 lock in share mode;


I am wondering why the first way does not get a S lock?