The Single Best Interview Question for Pretty Much Any Job in Tech

This question originally appeared on Quora: What is the best interview question ever? Answer by Wade Myers, Inc 5000 entrepreneur and managing director of Boldmore Growth Partners.

When it came time to interview candidates for programming, development, and strategy positions, there was one question I always loved to ask: Einstein’s Riddle. This brainteaser is a fantastic way to assess interviewees whose roles would require structured thinking and rigorous mental horsepower.

“Einstein’s Riddle” is a logic puzzle whose creation is often credited to a young Albert Einstein (though there is no hard evidence to support this claim), and it demands patience and logical processing to be properly solved. I would offer interviewees the question in writing and offer them a whiteboard and marker to show me their work.

Here’s the riddle in three pieces:

Background:

• There are five houses in five different colors
• A person with a different nationality lives in each house
• The five owners each drink a certain type of beverage, smoke a certain brand of cigar, and keep a certain pet
• No owners have the same pet, smoke the same brand of cigar, or drink the same beverage

Clues:

1. The Brit lives in the red house
2. The Swede keeps dogs as pets
3. The Dane drinks tea
4. The green house is on the left of the white house
5. The green homeowner drinks coffee
6. The person who smokes Pall Mall rears birds
7. The owner of the yellow house smokes Dunhill
8. The man living in the center house drinks milk
9. The Norwegian lives in the first house
10. The man who smokes Blend lives next to the one who keeps cats
11. The man who keeps the horse lives next to the man who smokes Dunhill
12. The owner who smokes Blue Master drinks beer
13. The German smokes Prince
14. The Norwegian lives next to the blue house
15. The man who smokes Blend has a neighbor who drinks water

The Question:

Who has fish for a pet?

Most applicants completely gave up in exasperation and didn’t even attempt to answer the riddle because they either thought it was impossible to answer, or they couldn’t seem to figure out that you just need to start digging in and nail down one detail at a time.

The few candidates that were really, really good with frameworks and logical processing wrote out a grid and continued to loop through the clues until they filled it in. The reason for the numerous loops is that many of the clues are not useful until you’ve filled in what you already know.

I had one 20-year-old candidate for a project manager position go to my white board and answer it in about three minutes flat. I was stunned. Not only did I hire him on the spot, but within 18 months he was the managing director of my entire European operation—he grew a beard to look older because he feared that our European clients would never want to work with a 21 year old managing director. By 22 years old he was at Stanford Business School getting his MBA (I wrote a letter of recommendation, so I get some credit, right?). He was truly in the top 2% of anyone I’ve ever met or worked with.

How to answer Einstein’s Riddle

The easiest way to solve this puzzle is start with a grid like this and begin filling it in with clues:

Clues 8 and 9 allow us to fill in cells directly:

Clue 14 places the blue house next to the Norwegian, and the green and white houses as 4 and 5:

Clue 1 says that the Brit lives in a red house, which rules out all but 3. By process of elimination, the first house is yellow, and clue 7 adds that the owner smokes Dunhills:

Clue 11 places horses next to the Dunhill house. The beer drinker smokes Blue Master, the Dane drinks tea, and milk and coffee are in houses 3 and 4. So the Norwegian—who smokes Dunhill—drinks water. And the water drinker’s neighbor smokes Blend. With the only beverages remaining being tea and beer, and knowing that the beer drinker smokes Blue Master, the second house has the tea drinking Dane:

The last beverage, beer, goes in house 5 with its Blue Masters. The German Prince-smoker must therefore be in House 4. This leaves the Swedish dog owner for House 5, then the bird-rearing Pall Mall smoker for House 3:

Since the cat owner lived next the Blend smoker, we can see that there is only one slot left: in the German’s house. So it is the German who keeps the fish:

If you have a candidate that can solve this, you need to hire them!