Answer to old chestnut: five men, houses, colors, nationalities, beverages, cigars, and pets:

Begin by filling into the grid all the things you are directly told (clues 1, 2, 3, 5, 6, 7, 8, 9, 12, 13), and eliminating the immediately impossible choices. You can also fill in 14 because together with clue 9 you know the blue house is the second one, and clue 4 eliminates a couple choices for house color/position, and for clues 10, 11, and 15 you can X out the junction of the two values named. In the grid below, I've labeled the two-word cigar brands with capital letters because some thoughtless person didn't choose ones that all started with different letters of the alphabet.

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit X???? XXOXX ???X? ????X ??X??
 Dane X???? ??X?? XXXOX ????X ??X??
Germ. X???? ??X?? ???X? XXXXO ??X??
Norw. OXXXX ??X?? ???X? ????X ??X??
Swede X???? ??X?? ???X? ????X XXOXX

 bird ????? ????? ????? XXXOX
  cat ????? ????? ????? X??X?
  dog ????? ????? ????? ???X?
 fish ????? ????? ????? ???X?
horse ????? ????? ????? ??XX?

blend ????? ????X X???X
 B.M. ????? ????X OXXXX
dunh. ????? XXXXO X????
 P.M. ????? ????X X????
prnc. ????? ????X X????

 beer ??X?? ?X???
coff. ??X?? XOXXX
 milk XXOXX ?X???
  tea ??X?? ?X???
water ??X?? ?X???

 blue XOXXX
green ?X??X
  red ?X???
white XX???
yell. ?X???

Now, for each O in the grid, anything else that is known for one of those variable values can be copied to the other. For instance, the blue house is the second one, and we know the blue house does not belong the Brit, the Dunhill smoker, or the coffee drinker, we know the second house also does not belong to those people.

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit XX??? XXOXX ???X? ??X?X ??X??
 Dane X?X?? ??X?? XXXOX ?X??X ??X??
Germ. X???? ??X?X X??X? XXXXO X?X??
Norw. OXXXX X?XX? ??XX? ????X ??X??
Swede X???? ??X?? ???X? ???XX XXOXX

 bird ????? ????X X???? XXXOX
  cat ????? ????? ????? X??X?
  dog X???? ????? ????? ???XX
 fish ????? ????? ????? ???X?
horse ????? ????X ????? ??XX?

blend ????? ????X X???X
 B.M. ??X?? ?X??X OXXXX
dunh. ?X??? XXXXO XX???
 P.M. ????? ????X X????
prnc. X???? ??X?X X??X?

 beer ??X?? ?X??X
coff. ?XX?? XOXXX
 milk XXOXX XX???
  tea X?X?? ?X???
water ??X?? ?X???

 blue XOXXX
green ?XX?X
  red XX???
white XX???
yell. ?X???

At this point you should check for rows or columns in the five-by-five subgrids that have 4 Xs and a blank cell; there are none, so try to extract more information from the remaining clues. Clues 4, 10, 11, and 15 have more information left to offer.

The Dunhill smoker does not live in the 2nd house, so the horse owner cannot live in the first house by clue 11 (and thus the Norwegian does not own the horse).

The green house can only be the first or fourth house, and the white house is just to its right, but cannot be the second house, so can only be the fifth house (and the green house is fourth). This forces the red house to be third and the yellow house first, and we can copy all information between color and house position.

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit XXOXX XXOXX ???X? ??X?X ??X??
 Dane X?X?? ??X?X XXXOX ?XX?X ??X??
Germ. X?X?? ??X?X X??X? XXXXO X?X??
Norw. OXXXX XXXXO ??XX? XXOXX X?X?X
Swede X?X?? ??X?X ???X? ??XXX XXOXX

 bird X???? ????X X???? XXXOX
  cat ????? ????? ????? X??X?
  dog X???? ????X ????? ???XX
 fish ????? ????? ????? ???X?
horse X???? ????X ????? ??XX?

blend X???? ????X X???X
 B.M. X?XX? ?XX?X OXXXX
dunh. OXXXX XXXXO XX???
 P.M. X???? ????X X????
prnc. X?X?? ??X?X X??X?

 beer X?XX? ?XX?X
coff. XXXOX XOXXX
 milk XXOXX XXOXX
  tea X?XX? ?XX?X
water ??XX? ?XX??

 blue XOXXX
green XXXOX
  red XXOXX
white XXXXO
yell. OXXXX

By elimination we know the water drinker lives in the first house (which is yellow and owned by the Norwegian Dunhill smoker). Since we now know the Dunhill smoker lives in the first house, the horse owner (by clue 11) lives in the second house (the blue one). Likewise, since we now know the water drinker lives in the white house, the Blend smoker lives in the second house (the blue one, and he is the horse owner).

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit XXOXX XXOXX ???XX X?X?X ??X?X
 Dane X?X?? ??X?X XXXOX ?XX?X ??X??
Germ. XXX?? X?X?X X??XX XXXXO X?X?X
Norw. OXXXX XXXXO XXXXO XXOXX X?X?X
Swede XXX?? X?X?X ???XX X?XXX XXOXX

 bird XX??? X???X X???X XXXOX
  cat ?X??? X???? ????? X??X?
  dog XX??? X???X ????X X?XXX
 fish ?X??? X???? ????? X??X?
horse XOXXX OXXXX ?XX?X OXXXX

blend XOXXX OXXXX X???X
 B.M. XXXX? XXX?X OXXXX
dunh. OXXXX XXXXO XXXXO
 P.M. XX??? X???X X???X
prnc. XXX?? X?X?X X??XX

 beer X?XX? ?XX?X
coff. XXXOX XOXXX
 milk XXOXX XXOXX
  tea X?XX? ?XX?X
water OXXXX XXXXO

 blue XOXXX
green XXXOX
  red XXOXX
white XXXXO
yell. OXXXX

By process of elimination, the Dane lives in the second house (which is blue, and he smokes Blend and owns a horse; we also know he drinks tea). Thus the beer drinker (who smokes Blue Master) lives in the fifth (white) house.

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit XXOXX XXOXX X??XX XXX?X ??X?X
 Dane XOXXX OXXXX XXXOX OXXXX XXXXO
Germ. XXX?X X?XXX X??XX XXXXO X?X?X
Norw. OXXXX XXXXO XXXXO XXOXX X?X?X
Swede XXX?? X?X?X ???XX X?XXX XXOXX

 bird XX??X X??XX X??XX XXXOX
  cat ?X??? X???? ???X? X??X?
  dog XX??? X???X ???XX X?XXX
 fish ?X??? X???? ???X? X??X?
horse XOXXX OXXXX XXXOX OXXXX

blend XOXXX OXXXX XXXOX
 B.M. XXXXO XXXOX OXXXX
dunh. OXXXX XXXXO XXXXO
 P.M. XX??X X??XX X??XX
prnc. XXX?X X?XXX X??XX

 beer XXXXO XXXOX
coff. XXXOX XOXXX
 milk XXOXX XXOXX
  tea XOXXX OXXXX
water OXXXX XXXXO

 blue XOXXX
green XXXOX
  red XXOXX
white XXXXO
yell. OXXXX

Further process of elimination fills in most of the rest of the grid.

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit XXOXX XXOXX XXOXX XXXOX OXXXX
 Dane XOXXX OXXXX XXXOX OXXXX XXXXO
Germ. XXXOX XOXXX XOXXX XXXXO X?X?X
Norw. OXXXX XXXXO XXXXO XXOXX X?X?X
Swede XXXXO XXXOX OXXXX XOXXX XXOXX

 bird XXOXX XXOXX XXOXX XXXOX
  cat ?XX?X X?XX? X?XX? XX?X?
  dog XXXXO XXXOX OXXXX XOXXX
 fish ?XX?X X?XX? X?XX? XX?X?
horse XOXXX OXXXX XXXOX OXXXX

blend XOXXX OXXXX XXXOX
 B.M. XXXXO XXXOX OXXXX
dunh. OXXXX XXXXO XXXXO
 P.M. XXOXX XXOXX XXOXX
prnc. XXXOX XOXXX XOXXX

 beer XXXXO XXXOX
coff. XXXOX XOXXX
 milk XXOXX XXOXX
  tea XOXXX OXXXX
water OXXXX XXXXO

 blue XOXXX
green XXXOX
  red XXOXX
white XXXXO
yell. OXXXX

Finally we can use clue 10. The Blend smoker lives in house 2, so the cat owner can only live in house 1 or 3, and only 1 is still possible.

      house color drink cigar pets
      12345 bgrwy bcmtw bBdPp bcdfh
 Brit XXOXX XXOXX XXOXX XXXOX OXXXX
 Dane XOXXX OXXXX XXXOX OXXXX XXXXO
Germ. XXXOX XOXXX XOXXX XXXXO XXXOX
Norw. OXXXX XXXXO XXXXO XXOXX XOXXX
Swede XXXXO XXXOX OXXXX XOXXX XXOXX

 bird XXOXX XXOXX XXOXX XXXOX
  cat OXXXX XXXXO XXXXO XXOXX
  dog XXXXO XXXOX OXXXX XOXXX
 fish XXXOX XOXXX XOXXX XXXXO
horse XOXXX OXXXX XXXOX OXXXX

blend XOXXX OXXXX XXXOX
 B.M. XXXXO XXXOX OXXXX
dunh. OXXXX XXXXO XXXXO
 P.M. XXOXX XXOXX XXOXX
prnc. XXXOX XOXXX XOXXX

 beer XXXXO XXXOX
coff. XXXOX XOXXX
 milk XXOXX XXOXX
  tea XOXXX OXXXX
water OXXXX XXXXO

 blue XOXXX
green XXXOX
  red XXOXX
white XXXXO
yell. OXXXX

In summary, the properties of the five men are (in order of their houses):

  1. Norwegian, cat, Dunhill, water, yellow
  2. Dane, horse, Blend, tea, blue
  3. Brit, bird, Pall Mall, milk, red
  4. German, fish, Prince, coffee, green
  5. Swede, dog, Blue Master, beer, white
The answer to the question posed is that the fish is owned by the German.

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