Spammers are one of the great plagues of the Internet.
CAPTCHAs are meant to distinguish humans from robots.
One of the cutest types of CAPTCHA is the math challenge: instead of numbers, a tiny arithmetic operation is written out in words, and the human must solve it.
The challenge here is to write code that allows a robot to solve this puzzle.
This kind of simplistic challenge is still proudly used by some modules for a well-known CMS.