Margin not working inside div block, collapse? How to fix?

Question

css

There is the following code

<div id=statistics>
     <div id=stats>
        <div>42 <span>WEB DESIGN PROJECTS</span></div>
        <div>42 <span>WEB DESIGN PROJECTS</span></div>
        <div>42 <span>WEB DESIGN PROJECTS</span></div>
        <div>42 <span>WEB DESIGN PROJECTS</span></div>
     </div>
</div>

I want to center the contents of the #stats block in the center of the #statistics block using margin, but it doesn't work, why? And how to fix it?
The code is here https://jsfiddle.net/w6q8jujf/1/

Answer 1

Example

Answer 2

So?
https://jsfiddle.net/w6q8jujf/3/