NSAIDs are painkillers, like Motrin, Advil or Aleve, that are commonly used to treat headaches and joint pain. They relieve pain by inhibiting one or both of the enzymes (COX-1 and COX-2) that produce metabolites called prostaglandins. This study will compare two NSAIDs – celecoxib and naproxen – to placebo (a pill that does not have any medication). We will measure the amounts of prostaglandins in your blood and urine to study how you respond to each of these NSAIDs. In the future, the information we learn in this study may help doctors choose which NSAID to prescribe to a specific patient so that he or she will have a good response and a low chance of side effects. There are 3 treatment phases during the study. During each treatment phase, you will take either celecoxib, naproxen, or placebo two times a day for 7 days. On the first day of each treatment phase, you will attend a baseline study visit at the Clinical and Translational Research Center (CTRC) at the Hospital of the University of Pennsylvania (~1 hour). On the last day of each treatment phase (Day 7), you will attend a whole-day study visit at the CTRC (~13 hours). You will attend a post-treatment study visit at the CTRC on each of the 3 days after completing the treatment phase (~1 hour/visit). You will then wait 2 weeks before taking the next study medication. The study visits will be repeated for the next treatment phase. When you have completed all 3 treatment phases, your participation in the study will end.