In the English language, some characters resemble others, especially when certain fonts are used. A common password algorithm scheme that arose in the early days of the Internet was to replace a character in a phrase with its look-alike.
The resulting phrase was called 'leet speak.' It is not a secure password scheme by any means, but it's fun to see phrases undergoing a 'leet' transformation. Enter your message, and our app will convert it to 'Leet Speak.' Try it!
Wikipedia has a good description of the Leet phenomenon.