If you've ever signed up for anything online, you've probably taken a CAPTCHA test. Maybe you deciphered some scrambled letters and numbers. Maybe you clicked on a bunch of pictures of stop signs. Or maybe you just clicked a box that said "I am not a robot."
These tests are one of the annoyances we put up with to do stuff on the Internet. But the story of CAPTCHA is shockingly interesting. It includes the rise of artificial intelligence, the quest to digitize millions of old books and newspapers, and a shady underworld of human beings paid to solve thousands of CAPTCHAs a day.