A CAPTCHA (IPA: /ˈkæptʃə/) is a type of challenge-response test use in computing to ensure that the response is not generated by a computer. The process involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. A common type of CAPTCHA requires that the user type the letters or digits of a distorted image that appears on the screen.