SPECIAL POST — One of the entries that solvers had the most trouble with last week was the acronym CAPTCHA.

A CAPTCHA is a program used to verify that a human, rather than a computer, is entering data. It stands for “Completely Automated Public Turing test to tell Computers and Humans Apart,” and they are commonly seen at the end of online forms to help thwart spam and automated extraction of data from websites. The term was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford of Carnegie Mellon University.

Text CAPTCHAs ask the user to enter text from a distorted image.