[jQuery] addClass only works once

[jQuery] addClass only works once


Im using the code below to display / hide form fields depending on
what value is selected from a drop down list (id='category'). the
optional form fields are all hidden by default when DOM is ready,
using
$(document).ready(function() {
    $("#dimensions").addClass("hidden")
    $("#size").addClass("hidden")
    $("#language").addClass("hidden")
    $("#inthebox").addClass("hidden")
    $("#color").addClass("hidden")
});
when i select "games", the p's with id 'dimensions' and 'inthebox'
appear - so far so good. when i then select "accessoires",
'dimensions' and 'inthebox' should get hidden by adding the class
'hidden' (.hidden{display:none;}) - but this never works! any ideas
why? firebug shows no errors ...
$(document).ready(function()
{
    $("#category").change( function()
        {
            if ($("#category").val() == "games")
            {
                $("#dimensions").addClass("show")
                $("#inthebox").addClass("show")
            }
            else if ($("#category").val() == "accessoires")
            {
                $("#dimensions").addClass("hidden")
                $("#inthebox").addClass("hidden")
            }
        });
});