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How does a CAPTCHA work?

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Quick answer

A CAPTCHA works by giving a quick challenge that's easy for humans but hard for bots, to keep automated programs out. Tasks like reading distorted text or picking images test for human perception that software has historically struggled to match.

Step by step

  1. It's a test to tell humans and bots apart.
  2. Classic versions show distorted text or image-selection puzzles.
  3. Humans solve them easily; simple bots can't.
  4. Modern ones analyze behavior (like mouse movement) invisibly.
  5. As AI improves, CAPTCHAs keep getting harder to design.

People also ask

What does CAPTCHA stand for?
'Completely Automated Public Turing test to tell Computers and Humans Apart' — a test only a human should pass easily.
Why do websites use CAPTCHAs?
To block bots from spamming forms, creating fake accounts, or scraping data, while letting real people through.
Why are CAPTCHAs getting harder?
As AI gets better at reading text and images, puzzles must get trickier — which is why many now quietly analyze your behavior instead.
How do CAPTCHAs work?
They present challenges humans solve easily but bots struggle with, sometimes analyzing behavior invisibly.
Why do some CAPTCHAs just need a checkbox?
They analyze signals like your mouse movement and history to judge you're human without a puzzle.

Part of the guide: How the Internet Works →