title: How do you put an element above another in mobile view only?
body: <p>I'm using Bootstrap 3.x and I have two divs of col-lg-6. Column X and Column Y.</p>
  
  <p>On desktop view, Column X is on the left and Column Y is on the right.</p>
  
  <p>How do I get it so that on smaller screen widths, Column Y is <strong>stacked above</strong> Column X?</p>