Image Captions on Hover

I'm trying to implement a feature for my website where hovering over an image shows its caption over the bottom of the image. Any tips on how to do this? I was trying to do it, but something in the css template I'm using was interfering.

Here are the images I want to put captions for

<section class="5grid is-gallery">
    <div class="row">
        <div class="4u"> <a href="matthewpiatetsky.com/MicroChess.html" class="image image-full"><img src="images/1a.jpg" alt=""></a>

        </div>
        <div class="4u"> <a href="matthewpiatetsky.com/ClusteringDistance.html" class="image image-full"><img src="images/2a.jpg" alt=""></a>

        </div>
        <div class="4u"> <a href="matthewpiatetsky.com/FourInARow.html" class="image image-full"><img src="images/3a.jpg" alt=""></a>

        </div>
    </div>
    <div class="row">
        <div class="4u"> <a href="matthewpiatetsky.com/Fidelity.html" class="image image-full"><img src="images/4a.jpg" alt=""></a>

        </div>
        <div class="4u"> <a href="matthewpiatetsky.com/Concordance.html" class="image image-full"><img src="images/5a.jpg" alt=""></a>

        </div>
        <div class="4u"> <a href="matthewpiatetsky.com/PhotoShare.html" class="image image-full"><img src="images/6a.png" alt=""></a>

        </div>
    </div>
</section>

Note: I tried to use these, not exactly sure what I was doing wrong. http://tympanus.net/codrops/2011/11/02/original-hover-effects-with-css3/

The code to achieve this would be something like this:

a {
  position: relative
}

a span {
  display: none;
  position: absolute;
  bottom: 0;
  width: 100%;
  padding: 10px;
  background: rgba(255,255,255,.8);
}

a:hover span {
  display: block
}

<a>
  <img src="http://placehold.it/300x200">
  <span>caption</span>
</a>