Cheryl's Enigma (Birthday)
Level 4 · Advanced · ●●●●○
Albert and Bernardo have just met Cheryl. Cheryl tells them: 'My birthday is one of these 10 days: May 15, May 16, May 19, June 17, June 18, July 14, July 16...
Pure logic
A collection focused on formal deduction: clear premises, crisp inference, and minimal ambiguity.
The two doors
Level 2 · Core · ●●○○○
You are in a room with two doors. One leads to freedom, the other to death. There are two guardians: one always tells the truth and the other always lies, bu...
The most difficult logic puzzle in the world
Level 5 · Expert · ●●●●●
Three gods A, B and C are called Truth, Falsehood and Random, in some order. Truth always tells the truth, False always lies, and Random answers randomly tru...
Trunks and padlocks without shared key
Level 3 · Intermediate · ●●●○○
You and I are far away and can only send trunks to each other by courier. Each one has their own padlock and their own key. If a trunk travels without a lock...
Missionaries and cannibals
Level 3 · Intermediate · ●●●○○
Three missionaries and three cannibals must cross a river with a boat that only carries a maximum of two people. On no shore can there be a group where the c...
The knight, the squire and the spy
Level 3 · Intermediate · ●●●○○
On an island there are three types of inhabitants: knights (they always tell the truth), squires (they always lie) and spies (they can lie or tell the truth)...
The elimination tournament
Level 3 · Intermediate · ●●●○○
A tennis tournament involves 1,024 players in a single-elimination format (the winner advances, the loser is eliminated). How many games do you need to play...
The island of blue eyes
Level 5 · Expert · ●●●●●
100 perfectly logical people live on an island. There are people with blue eyes and people with brown eyes, but no one knows what color their own eyes are (t...
The door that opens by itself
Level 4 · Advanced · ●●●●○
There is a safe with a 4-digit code (0000 to 9999). You can type a long sequence of digits. The box opens as soon as the last 4 digits entered match the code...
Synchronized sources
Level 4 · Advanced · ●●●●○
Three sources activate every 4, 6 and 9 minutes. At 12:00 all three were activated at the same time. A visitor enters between 12:00 and 12:30. Since entering...
The height shelf
Level 4 · Advanced · ●●●●○
Five books of different heights 1,2,3,4,5 (1 = shortest, 5 = tallest) are in a row. Each book writes down how many books taller than it are to its left. The...
The triple rotation
Level 4 · Advanced · ●●●●○
You start with: 1,\ 2,\ 3,\ 4,\ 5. The only operation allowed is to choose three consecutive cards and rotate them cyclically: abc\to bca\quad\text{o}\quad a...
The cursed chocolate (Chomp)
Level 5 · Expert · ●●●●●
There is a rectangular chocolate bar of m\times n squares, with m,n\ge2 . The box in the lower left corner is poisoned. Two players alternate turns. Each tur...
The magician and the five cards
Level 5 · Expert · ●●●●●
A spectator chooses 5 different cards from a standard deck of 52. The assistant sees the 5 cards, hands 4 to the magician (in the order he wants) and hides t...
The chain of lies (7 in a circle)
Level 3 · Intermediate · ●●●○○
There are 7 people sitting in a circle. Each one says exactly: “My neighbor on the right is a liar.” It is known that each person is of a unique type: or alw...
The party without different grips
Level 3 · Intermediate · ●●●○○
At a party there are n people, with n\ge 2 . Each person writes down how many handshakes they gave during the party. Question: Can n all different numbers ap...
Six people and a social triangle
Level 3 · Intermediate · ●●●○○
In a meeting of 6 people, exactly one of two things happens for each couple: or they know each other, or they don't know each other. Prove that at least one...