Problem with :contains function

I’m fairly new to Jquery now, but enjoying the early stages of what my tutorial book tells me. One thing which has confused me is the code below.

Now, I have googled a lot for a solution to my problem, but they all give the same solution, ie the one below, so I am wondering is it something to do with my computer, why it wont work?

I want it to apply the class/background colour to the text of “Cheese” in the page, but nothing happens in the browser when I test the page. I’ve got an alert firing on documentready, in case there was a code, error but it seems to ‘like’ the code without breaking the javascript on the rest of the page. I’ve even gone to lengths of copying an entire example code froma site and pasting it into a fresh file to test on my machine, but no joy.

 
$("#burgers span em:contains(Cheese)").css("background-color","red");

I can do other jquery stuff like animate, show/hides etc and I’m using v1.4.4

Your wisdom is appreciated.