CAPTCHA is often used as a security tool that blocks a computer pretending to be a human from getting into something, like a website. When you log into your favorite game site, you might be asked to identify a series of numbers, letters or objects in an image. That’s a CAPTCHA. The tasks are usually easy for humans, but difficult for computers, so they are great at making sure it’s a human logging on and not a computer.. Make your own CAPTCHA! Choose an image like a cat, a house or a traffic sign and make tricky versions of it.