.panel {

    transition: padding 0.5s;
    -moz-transition: padding 0.5s; /* Firefox 4 */
    -webkit-transition: padding 0.5s; /* Safari and Chrome */
    -o-transition: padding 0.5s; /* Opera */
    -ms-transition:padding 0.5s; /* IE9 (maybe) */     
  
}

.panel:hover {
  padding:10px;
}