They're called CAPTCHA puzzles and are designed to be difficult for computers to crack. Google's Street View technology, however, can decipher them with 99 percent accuracy.

CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. The term was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. The test is designed to tell humans and bots apart on the web for reasons such as minimizing spam.