You’ve experienced this dozens, if not hundreds, of times. Before being allowed to enter a website, you must type words written in a bizarre, distorted script inside a box. Dr. Luis von Ahn, a professor in the Computer Science Department at Carnegie Mellon University, estimates that people decipher script like this more than 200 million times a day. He should know. He invented the system. Captcha, as it is called, protects websites by demanding that visitors take a simple test that humans can pass but computers cannot. Captcha, in fact, is an acronym for Completely Automated Public Turing Test to Tell Computers and Humans Apart. It requires website visitors to interpret the text correctly and type the right letters before they can enter the site.