A particular inn always offers the same nine dishes on its dinner menu: A, B, C, D, E, F, G, H and I. Five foreigners arrive. Nobody tells them which dish corresponds to each letter and so they each select one letter without knowing what they will eat. The innkeeper arrives with the five dishes ordered and puts them in the center of the table so that they can decide who eats what. This goes on for two more nights. The foreigners, who are professors of logic, were able to deduce by the dishes they ordered which letter represents what dish. What could have been the dishes ordered each of the three nights? Note: Each dish was ordered at least once.
They could have ordered ABCDD their first night (finding out what D is), AEFGG the second night (finding out what G is and what A is, since they had ordered it the previous night too), and BEHII the third night (finding out what I, B and E are). This leaves C, F and H out, and since they had never ordered these dishes twice, and each came out on a different night, they should know what they are.
Today's brain teaser courtesy of Braingle.com.