My son has designed a t-shirt about one of his passions, aside from explosions, Fear Factor, and Court TV. If you go to Zazzle.com, you can make a nine-year-old very happy, even if you don't buy his shirt.
I could talk about the internet, personalization, one-one-many markets, but really, he just had a good time designing the shirt, and imagining the millions of people who are sure to buy it.