why the hover effect doesn't quit when i retire the mouse from the square?

why the hover effect doesn't quit when i retire the mouse from the square?

I wonder why, its necessary to write another code for mouse to leave?

$(document).ready(function() {
    $(".banner1").hover(function() {
        $(".banner1").animate({width:"100%"},1000);
         $(".banner1").css({'z-index' : '1'}); 
    }); 
});