When first confronted with this problem, I tried to solve it by writing and running a Prolog program. That did, however, not help me any further because this problem has lots of equivalences inside, and I simply did not know how to deal with equivalences using a Prolog implementation with depth-first search. Disillusioned, I turned off my computer and solved it with pen and paper, which took me about half an hour.

Situation:
There are five men. Each of these men lives in a house with a certain color, has a certain nationality, prefers a certain drink, has got a certain pet and smokes a certain brand of tobacco.

Nationalities	House colors	Drinks	Pets	Tobacco brands
British	blue	Beer	Bird	Dunnhill
Danish	green	Coffee	Cat	Marlboro
German	red	Milk	Dog	Pall Mall
Norwegian	white	Tea	Fish	Rothmann's
Swedish	yellow	Water	Horse	Winfield

Constraints:

• The Briton lives in the red house.
• The Swede has a dog.
• The Dane loves tea.
• The green house is located on the left of the white one.
• The person who owns the green house drinks coffee.
• The person who smokes Pall Mall has got a bird.
• The man who lives in the middle house is a milk-drinker.
• The owner of the yellow house smokes Dunnhill.
• The Norwegian lives in the left-most house.
• The man who smokes Marlboro lives next to the one that has got a cat.
• The man who owns a horse lives next to the one that smokes Dunnhill.
• The man who smokes Winfield likes beer.
• The Norwegian lives next to the blue house.
• The German smokes Rothmann's.
• The person who smokes Marlboro has a neighbor who drinks water.

Question:
Who owns the fish?