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Who is the knight?

On this island knights only tell the truth and knaves only lie.

You meet two locals, Mo and Jo.

Mo says: “We are both knaves.”

So which of the two is the knight? Lock in your gut answer.

← BlogSep 18, 20263 min read

12 logic puzzles that make you prove it

Heron

I'm Heron. I once spent a whole afternoon on a grid with four names and three drinks, and I got there by guessing, which is the one method that does not count.

So these twelve are the ones where guessing loses. Every answer is forced by the lines you were handed, plus what you already know about coins, bulbs, and calendars.

Assume it, then break it

A logic puzzle asks you to know almost nothing. It hands you a few lines and asks which world they leave standing.

When a line talks about the person saying it, test it by assuming it. Whichever assumption ends in nonsense is the one we throw out.

Let's run that on a fresh sentence. Mo has another go, and this time he says that at least one of them lies.

Step 1 of 6Mo: at least one of us lies

“one lies”

Different sentence, opposite pair: this time Mo is the knight, and Jo is the one lying.

Logic grid puzzles, where crossing out beats guessing

Five to warm up on, and the first two are grids with the grid left out. Four names and four drinks, then three finishers in a row: both fit in your head.

Grid and zebra puzzles run on the clues that say no. Each one crosses a box off the table, and a row with one box left hands you a fact for the next clue.

The island pair up top was riddle 1, so the puzzle cards pick up at 2. Keep a private tally as you go, because the scoreboard at the end wants a number from you.

Riddle 2 of 12

Ana, Ben, Cal, and Dee drink tea, soda, milk, and water. Ana has water, Cal milk, Ben never tea. Who has tea?

Riddle 3 of 12

Ana, Ben, and Cal finish first, second, and third. Ben beats Cal, and Ana is not first. Who won?

Riddle 4 of 12

All roses are flowers, and some flowers fade fast. Does it follow that some roses fade fast?

Riddle 5 of 12

Two coins add up to 30 cents, and one of them is not a nickel. What are the two coins?

Riddle 6 of 12

Three switches downstairs, one old filament bulb upstairs, one trip up. How do you tell which switch is its?
Heron

Two of those five die on a single word. The coins say one of them is not a nickel, most brains hear neither of them, and the roses die the same way on some.

We're calling that move The Ghost Clue: a line the puzzle never wrote, standing in your head with the real ones. The next six are built on it.

Logic puzzles for adults, and the clue that was never there

Six harder ones, and the arithmetic stays tiny throughout. Two people stuck on their own hats, four cards with a rule to break, and a house number that hides a tie.

One habit clears most of them: we write down what we were actually given. Anything in your head that survives the list is a Ghost Clue, and the same list is what cracks a hole with three honest measurements.

Riddle 8 is the famous one. Read its rule twice before you pick, because the rule only promises something in one direction.

Riddle 7 of 12

Three in a line see only ahead, hats from 2 black and 3 white. The back two cannot name their own. Front hat?

Riddle 8 of 12

Four cards read A, K, 4, and 7. Rule: a vowel on one side means an even number on the other. Turn which?

Riddle 9 of 12

Three kids: their ages multiply to 36 and add up to my house number. Why do I need one more clue?

Riddle 10 of 12

One of 1,000 juice barrels is spoiled, a sip shows it a day later, and sips can be mixed. Fewest tasters?

Riddle 11 of 12

The day before yesterday I was 17, and next year I will turn 20. What day is today, and my birthday?

Riddle 12 of 12

A boy and his dad crash. The surgeon says: “I cannot operate on him, he is my son.” Who is the surgeon?

Those four cards have been through the lab. Griggs and Cox took a rule their subjects had lived with, and improved performance was found, in a paper that mostly reports failures to replicate.

Speed does the rest of the damage. Shane Frederick built a famous three-question test out of the answer that arrives before the reading is done.

We named that reflex The Candy Trap, and the move-ordering puzzles in our math puzzles set feed on it just as happily.

What to remember

Heron
  • Assume it, then break it. A claim that contradicts itself under your own assumption has just answered the question for you.
  • Cross out with the clues that say no. Grids are won by the names a clue removes, and the survivor needs no clue of its own.
  • List what you were given. Everything else in your head is a Ghost Clue, and it is why the four cards keep winning.

Honest tally: how many did you solve without peeking?

FAQ

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