Distinguishing between you and a robot: You're trying to purchase an item online or log into an account. You enter your credentials, but before you proceed, you need to prove that you're a human user. Asking you to tick the "I'm not a robot" checkbox to confirm that you are, in fact, human seems curiously simple. For many users, these have become an annoying necessity of using the internet. For the companies using them, however, the cybersecurity technology has become a vital security measure that protects websites and users from spam, as this clip from Vox explains (watch, runtime: 8:03).

Why would anyone need to create a test that can tell humans and machines apart — apart from avoiding the next android replicant revolution, obviously? It's because of people trying to game the system — they want to exploit weaknesses in the computers running the site.

Which brings us to CAPTCHAs: In 2000, the first CAPTCHAs — short for Completely Automated Public Turing test to tell Computers and Humans Apart — were developed for Yahoo to prevent automated software programs (collectively called “bots”) from automatically setting up email accounts, which would in turn be used to pump out spam. Some bots might try to scan websites to steal email addresses or passwords. Other spambots might try to submit fake registrations or fraudulent sweepstakes entries. And that's just the tip of the iceberg. There's no end to the different types of malicious actions that bots can be programmed to do to wreak havoc on websites.

With a CAPTCHA, the goal is to create a test that humans can pass easily but machines cannot. CAPTCHAs weed out bots by presenting puzzles within the browser’s response that ostensibly only humans can solve and, therefore, eliminating bot submissions.

As the threats posed by bots have evolved, so have the CAPTCHA mechanisms intended to stop them. In the early days, users were asked to read slightly distorted text and type the correct characters to pass the test,and while initially successful, rapid advances in computing meant that bots were soon able to process the text as well as a human.

Say hello to reCAPTCHAs: CAPTCHAs integration with Google in 2009 and the introduction of reCAPTCHAs marked the next phase in the battle against the bots. These tests had the same goal but with a twist: The prompts were all digital scans of books. Because computers are not always able to identify words from a digital scan, humans would complete the security test while also helping to digitize books for the internet archive. “Hundreds of mns of CAPTCHAs are solved by people every day. reCAPTCHA makes positive use of this human effort by channeling the time spent solving CAPTCHAs into digitizing text, annotating images and building machine learning datasets. This helps preserve books, improve maps, and solve hard AI problems,” read Google’s website. But soon enough, bots got so good at passing a reCAPTCHA that by 2014 Google found that the tests could be deciphered by bots over 99% of the time.

In 2014, Google started phasing out the classic service. In place, it started asking you to tick the "I’m not a robot" checkbox, endearingly called the “No CAPTCHA reCAPTCHA.” In 2017, Google announced it was getting rid of No CAPTCHA. Instead the service would rely on techniques like noticing how you move an onscreen pointer or analyzing your browsing habits to determine whether you are human or robot. This is called “Invisible reCAPTCHA.” If Google is still unsure of your humanness after clicking the checkbox, you will be shown a visual reCAPTCHA (with words, street signs or images) as an additional security measure.

The next frontier: Because CAPTCHA is such an elegant tool for training AI, any given test is only ever going to be temporary. As AI learns and develops from existing technology, CAPTCHAs will again have to evolve. Recently there have been efforts to develop game-like CAPTCHAs: tests that require users to rotate objects to certain angles or move puzzle pieces into position, with instructions given not in text but in symbols or implied by the context of the game board. The hope is that humans would understand the puzzle’s logic but computers, lacking clear instructions, would be stumped. Other researchers have tried to exploit the fact that humans have bodies, using device cameras or augmented reality for interactive proof of humanity. However, particularly as artificial intelligence continues to improve, standalone visual-challenge-response approaches are becoming less viable.