CAPTCHA tests have been making internet users miserable for years. They're meant to weed out bots, but just about as often they're unreadable to humans. Google's reCAPTCHA service is better these days, simply asking you to check a checkbox, but now even that is more than Google wants users to see. CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart, and conveniently sounds like "capture." Google isn't getting rid of the test, though.  The CAPTCHA is going into stealth mode.

Instead of making you do something to prove you're a human, Google's new reCAPTCHA simply works in the background, identifying bots from humans without any manual intervention. Google is using "a combination of machine learning and advanced risk analysis that adapts to new and emerging threats" to make this work. They're intentionally being vague, though, as specifics are exactly what would let bot makers crack the code.

Back when reCAPTCHA was still asking us to punch in words or numbers, we were unwittingly working for Google. As Ars Technica explains, anytime a word came up, it was from Google's project to digitize millions of books. Their system would scan books and try to recognize the characters, but sometimes scanning artifacts or book conditions made it difficult for computers to make sense of the text. If a number came up, that was from Google Street View's car. The system knew it was looking at an address but couldn't read what it was. Things have come along far enough that that's no longer necessary.

How to know you're a replicant

If you do see a challenge from reCAPTCHA, that means Google thinks you might be a bot. In that case, you might want to check and make sure you're really human. There are some pretty good tests to determine this stuff:

Keep in mind that while reCAPTCHA is the most popular of these tests, it isn't the only one. You'll likely be asked to solve one of these tests by other sites that don't use Google's option. But every day we're seeing more and more just how deep Google's machine learning is going.