... Eaten in their place of historical origin.

 It makes your brain and stomach happ

Much of this rule is covered by the fact that foods consumed while on vacation have no calories. But, to add a clause to the original vacation rule, calories consumed in their place of historical origin are definitely calorie free. Visiting the Guinness factory in Dublin? Or the Jelly Belly Factory in California? Have no fear! The calories in your beer and candy don't count, mostly because they are being consumed for educational purposes. After a tour of the facilities teaches you about the history and production of your favorite foods and drinks, sampling the product is definitely necessary. All calories consumed during this learning exercise are immediately used up by your brain in its attempt to absorb and retain all the information your are receiving, and are thereby classified as "brain food."

You might even need a second round of samples to be sure you are learning all you can!