craigslist uses the reCAPTCHA service from Google to help verify that a real person is posting an ad.

When asked to "type the words below," you should see images/phrases that have been retrieved from Google's servers.

If no words appear, here are several things to consider:

• you didn't give the page long enough to load -- try reloading as it can take up to 10 seconds at times to load the images from Google's servers

• you are using a browser extension like NoScript, which disables some scripts from running -- either turn off javascript entirely or tell the browser extension that it's OK to run scripts from google.com

• you are using a browser extension like RequestPolicy, which prevents page elements from being loaded from other sites -- tell RequestPolicy it's all right to load items from google.com when viewing that page

• make sure you can access this link directly (http://www.google.com/recaptcha/demo/) -- if you are unable to do so, your computer cannot access data from google.com for some other reason (possibly a network block from your network provider or block by third-party software like Norton Internet Security)

Click here for more info (and further troubleshooting tips) from Google on their reCAPTCHA service.