Help a newbie with a puzzle

Hi, ihave a problem understanding this puzzle can anyone help?

the solution is supposed to be Ko
where is my fault on this sequence ( sorry if i am using wrong notation , i am just a chessplayer)

1.S19 S18
2. T18 and this is supposed to be Ko. Lets say Black plays something irrelevant.Then white can continue wtih 3 T19 and if black plays R19 white goes T19 and black cannot make two eyes and his group is therefore dead , what am i doing wrong?

You thinking is correct. Just one thing.

The definition of ko is that black wont play something irrelevant. He will play a ko threat.

http://senseis.xmp.net/?Ko

yup , thank you for clearing this out to me