title: Practical non-image based CAPTCHA approaches?
body: <p>It looks like we'll be adding <a href="http://en.wikipedia.org/wiki/Captcha" rel="noreferrer">CAPTCHA</a> support to Stack Overflow. This is necessary to prevent bots, spammers, and other malicious scripted activity. We only want human beings to post or edit things here!</p>
  
  <p>We'll be using a JavaScript (jQuery) CAPTCHA as a first line of defense:</p>
  
  <p><a href="http://docs.jquery.com/Tutorials:Safer_Contact_Forms_Without_CAPTCHAs" rel="noreferrer">http://docs.jquery.com/Tutorials:Safer_Contact_Forms_Without_CAPTCHAs</a></p>
  
  <p>The advantage of this approach is that, <strong>for most people, the CAPTCHA won't ever be visible!</strong></p>
  
  <p>However, for people with JavaScript disabled, we still need a fallback and this is where it gets tricky.</p>
  
  <p>I have written a <a href="http://www.codeproject.com/KB/custom-controls/CaptchaControl.aspx" rel="noreferrer">traditional CAPTCHA control for ASP.NET</a> which we can re-use.</p>
  
  <p><img src="https://i.stack.imgur.com/Puvbf.jpg" alt="CaptchaImage"></p>
  
  <p>However, I'd prefer to go with something textual to avoid the overhead of creating all these images on the server with each request.</p>
  
  <p>I've seen things like..</p>
  
  <ul>
  <li>ASCII text captcha: <code>\/\/(_)\/\/</code></li>
  <li>math puzzles: what is 7 minus 3 times 2?</li>
  <li>trivia questions: what tastes better, a toad or a popsicle?</li>
  </ul>
  
  <p>Maybe I'm just tilting at windmills here, but I'd like to have a less resource intensive, non-image based <code>&lt;noscript&gt;</code> compatible CAPTCHA if possible.</p>
  
  <p>Ideas?</p>