What causes a headache?

Indeed, sir, the causes of a headache are as varied as they are numerous. The most common culprit is tension, that familiar tightness around the forehead or at the back of the head, often brought on by stress or prolonged concentration. Another frequent offender is migraine, which may bring with it a throbbing pain and sometimes nausea or sensitivity to light. One might also consider other factors, such as dehydration, lack of sleep, or even the lingering effects of a particularly spirited conversation over tea. In some cases, a more persistent or severe headache could signal something more serious, but I daresay most are content with a little rest and perhaps a pot of good Earl Grey.