How can a chicken with its head cut off run around? I've seen it
happen once when my cousin killed a chicken that he had, although it
was so long ago and so unlikely that I thought that my memory is
playing tricks on me. But apparently, not.
http://dictionary.reference.com/search?q=like%20a%20chicken%20with%20its%20head%20cut%20off

some funny story............

http://home.nycap.rr.com/useless/headless_chicken/

Here's one theory:

"Birds (and other dinosaurs) possess much larger ganglia (nerve bundles) in
their hip and shoulder regions than do mammals.  It's pretty well known that
when a chicken's head is cut off, the body still possesses the ability to
flap and run.  What I have been wondering about is if these ganglia act like
regional 'co-processors' that off-load some of the motor control from the
brain.  I don't suggest that birds and dinosaurs 'think' with these areas,
but that a level of gross motor control may occur there requiring only simple
commands from the brain like 'run' or 'fly fast.'  This type of operation
would be especially useful in animals like birds because it would mean the
brain could be smaller and the weight from an advanced nervous system could
be distributed more evenly and not make the bird quite so top-heavy." 

http://dml.cmnh.org/1995Jun/msg00787.html

Another theory:

"American scientists found accumulation of nerval tissue on the inner
surface of the gullet and the stomach. Scientists counted that the
number of neurons in the 'third brain center' made up over 100
million, which is more than even in the spinal cord. They say, it is
not a mere nervous joint, but a formation capable of keeping
information and even influencing our emotions and health. American
scientists say, 'the abdominal brain' commands secretion of hormones
in stress situations, it urges the body for struggle or escape.
However, the phenomenon isn't thoroughly studied; it is not ruled out
that it may unveil the mystery of decapitated bodies that can run
after execution."

http://www.wintersteel.com/Life_Without_Brain.html