he old reCAPTCHA system was pretty easy -- just a simple "I'm not a robot" checkbox would get people through your sign-up page. The new version is even simpler, and it doesn't use a challenge or checkbox. It works invisibly in the background, somehow, to identify bots from humans. Google doesn't go into much detail on how it works, only saying that the system uses "a combination of machine learning and advanced risk analysis that adapts to new and emerging threats." More detailed information on how the system works would probably also help bot-makers crack it, so don't expect details to pop up any time soon. When sites switch over to the invisible CAPTCHA system, most users won't see CAPTCHAs at all, not even the "I'm not a robot" checkbox. If you are flagged as "suspicious" by the system, then it will display the usual challenges.