a. Why do so many charitable activities like marathons, walks, and 5K runs give the participants "free" T-shirts, wristbands, hats, bumper stickers, and so forth?
b. Charitable organizations could probably make a lot of money for their cause by selling these items on their Web sites, but you usually have to actually attend the "2015 Cancer Run" to get the "2015 Cancer Run" T-shirt. Why?