At some McDonald's I have visited, they advertise that all the soft drinks are the same price. Small, medium and large sodas are all one dollar. (Price may vary in your locale).
I have no idea what the logic is behind this idea. Why would anyone order a small if they can get large for the same price? Or, would they say "I'm not really thirsty today, I will just order a medium.."
Can you really be sure how thirsty you will be? I think most people would err on the safe side and order the largest soda they had for the same price. I suppose I could hang around McDonald's (more than I already do) and watch what people order and take a survey:
"Hey stupid, did you know you get 20 more ounces of that poorly mixed, funny tasting crap for free?"
What really fries my burger is that if order the combo, you automatically get a medium. You have to pay extra to get a large.
If you figure this out, or if you have some inside information, drop me a line at firstname.lastname@example.org.