[jQuery] changing class

hi

i'm trying to change the class of a label and i'm using this:

$('label').click(function(){

        $(this).toggleClass('pop').toggleClass('tart');

        return false;
 });

initially the label class is set to 'pop'. when the label is clicked
it toggles pop off and tart on, but then it runs a 2nd time and
toggles it back to pop. why does the function run twice?