Why does this CSS margin-top style not work?

Question

I try to add margin values on a div inside another div. All works fine except the top value, it seems to be ignored. But why?

What I expected:
<

Answer 1

If you add any padding to #outer, it works.

Demo

#outer {
    width:500px; 
    height:200px; 
    background:#FFCCCC;
    margin:50px auto 0 auto;
    display:block;
    padding-top:1px;
}