catch yourself,
Tynenol, generically called acetaminophen, acts by inhibiting the cyclooxegenase-2 (COX2) pathway within the body. Without this COX2, the body cannot synthesize the necessary arachidonic acid metabolites for inflammation. This inflammation, a humoral response by the body to fend off infection, is what causes headaches. So inhibition of the mediators of headaches prevents (or relieves) headaches.
Now you know.