An alternate form of CAPTCHA human authentication that requires a person to click a check box next to a graphic that says "I Am Not a Robot. It may not be all that difficult to program a bot that recognizes this image and programmatically selects the check box. However, in conjunction with the display of "I Am Not a Robot," the CAPTCHA is analyzing mouse movement and other conditions that would verify a person is actually making the assertion. See CAPTCHA
An Additional Step
After clicking "I'm not a robot," an image like this (top) is often displayed requiring the user to identify which squares contain a particular object, such as a traffic light in this example.