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Here is a clearer example.

jQuery comes with WordPress, but remember to change $ to jQuery in the javascript, like this:

jQuery(document).ready(function(){

jQuery('li.headlink').hover(

function() { $('ul', this).css('display', 'block'); },

function() { $('ul', this).css('display', 'none'); });

});

It’s to prevent clashes with other javascript libraries.