child div expanding out of parent div issue - css

Question

I want something like this, click here.
This is a simple code in my HTML:
#mainContent {
margin:0;
width:100%;
height:600px;
padding: 0 0 30px 0;
}
#mainContent #sidebar { /* sidebar inside #mainContent */
float:left;
width:220px;
height:560px;
margin:10px 10px 40px 10px;
padding:10px 5px 10px 5px;
background-color:#CCCCCC;
}
#mainContent #featContent {
height:560px;
margin:10px 10px 40px 260px;
padding:10px 5px 10px 5px;
background-color:#CCCCCC;
overflow:auto;
}
<div id="mainContent">
<div id="sidebar"></div>
<div id="featContent"></div>
</div>
The problem is one of the divs are improperly placed .The #featContent div is going out of its parent #mainContent div for reason I don't understand. Check out this in jsFiddle here. The margin of #featContent goes out of the #mainContent bounds.

Answer 1

Demo
Add the following style to the #mainContent
#mainContent {
overflow:hidden;
}
Live demo

Answer 2

Add padding to parent div
#mainContent {
margin:0;
width:100%;
height:auto;
padding:10px 5px 40px 5px; background-color:red
}
#mainContent #sidebar { /* sidebar inside #mainContent */
float:left;
width:220px;
height:560px;
margin:10px 10px 40px 10px;
background-color:#CCCCCC;
}
#mainContent #featContent {
height:560px;
margin:10px 10px 40px 260px;
background-color:#CCCCCC;
overflow:auto;
}​
Demo hrer http://jsfiddle.net/j4C7T/