Wikipedia:
A headache is a condition of pain in the head; sometimes neck or upper back pain may also be interpreted as a headache. It ranks amongst the most common local pain complaints.
Headaches have a wide variety of causes, ranging from eye strain, sinusitis, migraine and tension to life-threatening conditions such as encephalitis, meningitis, cerebral aneurysms and brain tumors (although more than 95% of headaches are not life-threatening)
http://en.wikipedia.org/wiki/Headache
http://en.wikipedia.org/wiki/HeadOn
http://www.youtube.com/watch?v=f_SwD7RveNE
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