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The unwritten rule that comes into effect when McDonald's temporarily changes the price of all fountain soft drinks to $1 for summer.

The rule states that one MUST purchase a large-sized drink, regardless of the situation. In theory, this is to avoid possible fear of needing to drink more and teasing. Some may even buy a large drink simply because they are afraid of not getting every penny's worth of soda.
A: Hold on Jeff, let me go get a large coke.
B: I thought you were trying to lose weight.
A: Hey, it's cheap, and that's why I'm getting one.
B: Why get a large though?
A: Dude, it's the McSummer rule!
B: Whatever. You're not actually gonna drink the whole thing are you?
A: 'course not........just don't question it.
by drinkupordie June 21, 2010