Maybe you haven't heard of ReCAPTCHA. Perhaps for some reason, this verification code does not appear much in China, but I think you should have seen or used it more or less.

At this time, as long as we click the check box in front, the verification code algorithm will first do a security check with its "risk analysis engine". If it passes the direct inspection, we will get the following results directly.

If the algorithm detects that there are risks in the current system, such as unfamiliar network environment or simulation program, it needs to be checked twice. It will further pop up something like the following.

After verification, we can complete the submission of forms, such as login and registration. This verification code is Google's ReCAPTCHA V2 verification code, which belongs to a behavior verification code. These behaviors include clicking the check box, selecting the corresponding picture, voice dictation and so on. Only by verifying these behaviors can this verification code pass verification.