Change divs width/height dinamically : <p>Hi stackoverflow community! I'm making some practice with css/html5 "copying" other websites layouts and stuff to learn and understand how things works.</p>

<p>Now I'm get into this website codepen.io/picks/10/ where the divs after "Picked Pens" change their size dinamically if i resize my browser. So my question is: what is the "magic" behind this thing? 
I'm not looking for any code but just some stuff to study to understand how this kind of things works.</p>
