How to get a div with z-index to overlay another div on hover

I have a container div with several smaller div:s inside, all of them with float:left;. If I hover one of the smaller div:s the height and width should be increased and it should overlay the other div:s without moving them.

HTML:

 <div id="boxcontainer">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
 </div>

CSS:

.box {
  float:left;
  position: relative;
  width:150px;
  height:150px;
  margin:5px;
  background-color: yellow;
}

#boxcontainer {
  position: relative;
  width:500px;
  height:auto;
}

.box:hover {
  z-index:100;
  width:300px;
  height:auto;
}

How can I achieve this?

Solution

z-index only works on positioned elements (position:absolute, position:relative, or position:fixed).

Instead of float left try position absolute.

I have added a container around each box and positioned each element absolutely within it. This way you can add as many boxes as you wish and keep the same class.

EXAMPLE

HTML

<div id="boxcontainer">
    <div class="box">
        <div class="Inside"></div> 
    </div>         
    <div class="box">
        <div class="Inside"></div> 
    </div>         
    <div class="box">
        <div class="Inside"></div> 
    </div>     
</div>

CSS

#boxcontainer{
    position: relative;
    width:500px;
    height:500px;
}
.box{
    position:relative;
    float:left;
    width:150px;
    height:150px;
    margin:5px;
}
.Inside{    
    background:green;
    position:absolute;
    top:0;
    left:0;
    width:150px;
    height:150px;
    transition:all 0.5s ease-in-out;
    -webkit-transition:all 0.2s ease-in-out;
}
.Inside:hover{
    z-index:100;
    width:250px;
    height:250px;
    background:#666666;
}

http://jsfiddle.net/JJ3v4/3/