Square Illusion

    
    Check out this Pen!
<div class="container">
    <div class="square  square-1"></div>
    <div class="square  square-2"></div>
    <div class="circle  circle-1"></div>
    <div class="circle  circle-2"></div>
    <div class="circle  circle-3"></div>
    <div class="circle  circle-4"></div>
</div>


On Your Marks, Get Set, Wait content-box vs. border-box