Assign random class to list on click?

Question

JavaScript

Each new page (list) load assigns a random class to an N-list element...

var random = Math.floor(Math.random() * 1000);
var $li = $("#gallery li");
$li.eq(random % $li.length).addClass("on");

But how is it the same, only a random class is assigned by clicking on “I'm lucky!”, each time changing the value to a new one, and “Reset” will delete this class?
Edit jsfiddle