Why did kamikaze pilots wear helmets? "You can offer some cost benefit explanation," says Frank. "Maybe there's going to be turbulence on the way to the target, and [the helmet] will help him get there if he's protected. Maybe the target won't be there, and he'll have to come back - you'll want to keep him alive for another mission. I think more persuasive is that kamikaze pilots are first and foremost pilots, and pilots wear helmets. That's part of their identity as pilots. There are some interesting studies about the economics of personal identity." If, adds Frank, people can't function in the world without a "consistent, coherent personal identity … then that's part of economics too."

Another of your students asked why there is Braille on the keys of drive-through ATMs. So the manufacturer only has to produce one type of key. "The short, sensible answer is that it's cheaper to do it that way," he says.

Sounds reasonable. Now, brand extensions are all the rage at the supermarket these days. Doesn't it dilute loyalty to the core brand when a company introduces the same product in 15 different flavors or scents? How many kinds of Tide does the world really need? "In a world where everyone is different," says Frank, "it would be nice for everyone to get what she wanted. It's just an extension of that. It doesn't really cost much to add this extra variety. In the cases you're talking about, it's essentially the cost of printing new labels."