A Captcha or a challenge system, is a test to see if you are human.

1. Captcha And Challenge System
Tests Are Too Hard

2. Captcha stands for "Completely Automated Public Turing
test to tell Computers and Humans Apart". The phrase
was first used around 2000 by Luis von Ahn, Manuel
Blum, Nicholas Hopper, and John Langford of the
Carnegie Mellon University.
Turing means a repeatable test of a computer's ability to
act with human-like intelligence. If a computer can fool a
human into thinking they are communicating with another
human, the machine passes the test. The test was
introduced by Alan Turing in his 1950 paper, "Computing
Machinery and Intelligence".

3. A Captcha or a challenge system, is a test to see if you are
human. It is most often text, pictures, or sounds, that the
automated computer or robots cannot understand. In the
beginning, such tests screened out people with vision
problems, hearing problems, small children, and people
with mental challenges.
Now, many Captcha or challenge systems only allow
people with perfect vision, perfect hearing, and people
who can figure out a specific computer puzzle. In the old
(pre-2007) days, the tests were usually simple.

4. Simple is what is needed. When one visits Captcha.net,
they see a simple example that almost anyone can solve.
The problem is, that these days, the actual tests are
designed to defeat optical character readers (OCRs).
The words or numbers are garbled so much that the
average person has to retry a few times.

5. When I say garbled, I mean not even a teenager can figure out
what it says most of the time. It is time to move back to simple
Turing tests such as adding two numbers, or a simple
distortion of some text, or asking someone to pick all the
names from a list of words.
To make matters worse, great effort went into making an
alternative for people with vision problems. There is often an
"audio" button where you can listen instead of read. Try it
sometime, and see for yourself how "understandable" the
audio is to you. And of course, you cannot be on the phone,
on Skype, watching TV, or listening to anyone or anything
else while taking audio tests on web sites.

6. One benefit of so many web sites using Captcha or other
challenge systems, is that at least one company, reCaptcha,
used test results to help improve OCR on computer systems
that scan and read old books and text. Google bought
reCaptcha in 2009. Good for them, let's make sure it is good
for web surfers too.
When Captcha or challenge systems are too difficult, most
people must try several times, and people with vision
problems - forget about it. That is not right.

7. Resources:
http://deathbycaptcha.com
http://ezinearticles.com/?Captcha-And-Challenge-
System-Tests-Are-Too-Hard&id=6539639

8. Thank you for reading!