What’s in it?
My two older kids have recently learned that there is nutritional information on the packaging of the food they eat. Contrary to the intent of the information, however, they use it to determine which foods are unhealthy enough to be worth buying with their allowance. After all, they wouldn't want to waste their money on something that turned out to be healthy, so it's become something of a sport with them to find the food with the most sugar, salt, and calories.
I discouraged this practice at first, because I'd really prefer their eating choices to be determined by something besides trying to find the least healthy foods possible. After a time, however, I realized that they were actually learning something from the game. By searching for the least healthy treats, they were discovering exactly what was in the food they were eating, and it has become much easier to explain why they can't eat this kind of food on a regular basis.
It has also encouraged them to ask questions about the food that we do eat regularly. They want to know how much salt is really in these sausages that taste so salty. They want to know how much calories is in the cheese sauce that I make for them. They want to know why the sugar in fruit is better than the sugar in candy. I can't always answer these questions for them definitively, of course, but it has been interesting to have these conversations with them anyway.
Maybe in time they'll even learn to spend their allowance on healthier food.