<p>What is the best unobtrusive CAPTCHA for web forms? One that does not involve a UI, rather a non-UI Turing test. I have seen a simple example of a non UI CAPTCHA like the <a href="http://www.asp.net/AJAX/AjaxControlToolkit/Samples/NoBot/NoBot.aspx" rel="noreferrer">Nobot</a>  control from Microsoft. I am looking for a CAPTCHA that does not ask the user any question in any form. No riddles, no what's in this image.</p>

[{'answer_id': 3052, 'author': 'sestocker', 'author_id': 285, 'author_profile': 'https://Stackoverflow.com/users/285', 'pm_score': 2, 'selected': False, 'text': '<p>I think you might be alluding to an "invisible" captcha. Check out the Subkismet project for an invisible captcha implementation.</p>\n\n<p><a href="http://www.codeplex.com/subkismet" rel="noreferrer">http://www.codeplex.com/subkismet</a></p>\n'}, {'answer_id': 3204, 'author': 'Thejesh GN', 'author_id': 482, 'author_profile': 'https://Stackoverflow.com/users/482', 'pm_score': 2, 'selected': False, 'text': '<p>Try <a href="http://akismet.com/" rel="nofollow noreferrer">akismet</a> from wp guys </p>\n'}, {'answer_id': 3207, 'author': 'GateKiller', 'author_id': 383, 'author_profile': 'https://Stackoverflow.com/users/383', 'pm_score': 2, 'selected': False, 'text': '<p>I think asking the user simple questions like:</p>\n\n<p>"How many legs does a dog have?"</p>\n\n<p>Would be much more effective that any CAPTCHA systems out there at the moment. Not only is it very difficult for the computer to answer that question, but it is <strong>very easy for a human to answer</strong>!</p>\n'}, {'answer_id': 3241, 'author': 'ZombieSheep', 'author_id': 377, 'author_profile': 'https://Stackoverflow.com/users/377', 'pm_score': 1, 'selected': False, 'text': '<p><a href="http://meyerweb.com" rel="nofollow noreferrer">Eric Meyer</a> implemented a very similar thing as a WordPress plugin called <a href="http://meyerweb.com/eric/tools/wordpress/wp-gatekeeper.html" rel="nofollow noreferrer">WP-GateKeeper</a> that asks human-readable questions like "What colour is an orange?".  He did have some issues around asking questions that a non-native English speaker would be able to answer simply, though.  </p>\n\n<p>There are a few posts on <a href="http://meyerweb.com" rel="nofollow noreferrer">his blog</a> about it.</p>\n'}, {'answer_id': 3290, 'author': 'ZombieSheep', 'author_id': 377, 'author_profile': 'https://Stackoverflow.com/users/377', 'pm_score': 1, 'selected': False, 'text': '<p>@KP</p>\n\n<p>After your update to the original question, the only real option available to you is to do some jiggery-pokery in Javascript on the client.  The only issue with that would be provicing graceful degredation for non-javascript enabled clients.  </p>\n\n<p>e.g. You could add some AJAX-y goodness that reads a hidden form filed value, requests a verification key from the server, and sends that back along with the response, but that will never be populated if javascript is blocked/disabled.  You could always implement a more traditional captcha type interface which could be disabled by javascript, and ignored by the server if the scripted field if filled in...</p>\n\n<p>Depends how far you want to go with it, though.  Good luck</p>\n'}]
['https://Stackoverflow.com/questions/3027', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/439/']