[jQuery] firebug error: $ is not defined

[jQuery] firebug error: $ is not defined


I just got the Learning jQuery book from amazon delivered yesterday.
I am trying out the first chapter. When add the alice.js on the html
file and open it into the browser, I get this error in firebug:
$ is no defined
my alice.js file has the following content:
$(document).ready(function() {
$('.poem-stanza').addClass('emphasized');
});
any suggestions to fix it and emphasize is not working as claimed by
the book (no boxes..etc)?