How to flip div onclick?

.panel {
    margin: 0 auto;
    width: 130px;
    height: 130px;
    position: relative;
    font-size: .8em;
    -webkit-perspective: 600px;
    -moz-perspective: 600px;
}
/* -- make sure to declare a default for every property that you want animated -- */

/* -- general styles, including Y axis rotation -- */
 .panel .front {
    position: absolute;
    top: 0;
    z-index: 900;
    width: inherit;
    height: inherit;
    text-align: center;
    -webkit-transform: rotateX(0deg) rotateY(0deg);
    -webkit-transform-style: preserve-3d;
    -webkit-backface-visibility: hidden;
    -moz-transform: rotateX(0deg) rotateY(0deg);
    -moz-transform-style: preserve-3d;
    -moz-backface-visibility: hidden;
    /* -- transition is the magic sauce for animation -- */
    -o-transition: all .4s ease-in-out;
    -ms-transition: all .4s ease-in-out;
    -moz-transition: all .4s ease-in-out;
    -webkit-transition: all .4s ease-in-out;
    transition: all .4s ease-in-out;
}
.panel.flip .front {
    z-index: 900;
    -webkit-transform: rotateY(180deg);
    -moz-transform: rotateY(180deg);
}
.panel .back {
    position: absolute;
    top: 0;
    z-index: 800;
    width: inherit;
    height: inherit;
    -webkit-transform: rotateY(-180deg);
    -webkit-transform-style: preserve-3d;
    -webkit-backface-visibility: hidden;
    -moz-transform: rotateY(-180deg);
    -moz-transform-style: preserve-3d;
    -moz-backface-visibility: hidden;
    /* -- transition is the magic sauce for animation -- */
    -o-transition: all .4s ease-in-out;
    -ms-transition: all .4s ease-in-out;
    -moz-transition: all .4s ease-in-out;
    -webkit-transition: all .4s ease-in-out;
    transition: all .4s ease-in-out;
}
.panel.flip .back {
    z-index: 1000;
    -webkit-transform: rotateX(0deg) rotateY(0deg);
    -moz-transform: rotateX(0deg) rotateY(0deg);
}