Head Hurts

From How Stuff Works:

One common question about aspirin and other medicines is, "How does it know how to get to where the pain is?" The answer is that it doesn't! When you take aspirin, it dissolves in your stomach or the next part of the digestive tract, the small intestine, and your body absorbs it there. Then it goes into the bloodstream and it goes through your entire body. Although it is everywhere, it only works where there are prostaglandins being made, which includes the area where it hurts.

You may ask, "How come I have to keep taking aspirin if it works so well?" As with almost all chemicals, your body has ways of getting rid of aspirin. In this case, your liver, stomach, and other organs change aspirin to... surprise! Salicylic acid! This chemical then slowly gets changed a bit more by the liver, which sticks other chemicals onto the salicylic acid so that your kidneys can filter it out of your blood and send it out in your urine. This whole process takes about four to six hours, so you need to take another pill at that time to keep the effect going.

Ok, so I am consumed with selling our car right now, plus feeling happily overwhelmed about all the things I don't know about photography.

Here is the full article about aspirin.