avail. It didn't last long -- for most of the last 70 years, Red Cross doughnuts have remained free -- but veterans haven't forgotten. Chalk it up to something called categorical change, says Uri Simonsohn, a University of Pennsylvania business professor. Price changes, people can adjust to. But this was different. "Imagine, for Thanksgiving, you go to your parents' for dinner and after a nice dinner they say, 'That's going to be $10 per person,' " Simonsohn says. "You would be upset." The problem isn't the price -- $10 for a good turkey dinner might not be such a bad deal -- but that you're being charged in the first place. It changes the relationship. For the veterans, the Red Cross went from being a little like Mom, to being the corner store. So for all the Internet companies out there looking to charge for your service -- take heed. Changing categories is really difficult. Imagine you could call up a friend and say, "Meet me at the bar and tell me what's going on with the economy." Now imagine that's actually a fun evening. That's what we're going for at Planet Money . Want to know more? Check out our "about" page . Want to connect with the Planet Money team? Send us an email .