Overview
Published in Life International in 1962 and often credited to Einstein, this is the archetype of the constraint-satisfaction puzzle: fourteen sentences that squeeze twenty-five unknowns down to exactly one arrangement.
How to solve Einstein's Zebra Puzzle
- Draw a 5 × 5 table and only ever write a value when a clue forces it. Start with the absolute clues: Norwegian in house 1, milk in house 3.
- Use the adjacency clues to place the blue house, then the ivory/green pair — the milk clue kills the (3,4) option.
- From there each remaining clue eliminates exactly one possibility at a time until the table is full. No guessing is ever required.
The key insight
The puzzle is a hand-run constraint propagator. Every step is 'this value has exactly one legal home left', which is precisely the unit-propagation rule a SAT solver applies millions of times per second.
Variations & echoes
- Sudoku, Latin squares and n-queens are the same family; all three fall to propagate-then-backtrack.
- Written as a logic program the whole puzzle is a dozen lines of Prolog or MiniZinc and solves instantly.
Frequently asked questions
Did Einstein really write it?
Almost certainly not, and the claim that only 2% of people can solve it is folklore too. It is a well-built puzzle that needs patience, not genius.