A Coin at the Center
A coin seems to leave plenty of room around it. Once the centers are joined, that room becomes a full turn divided into minimum angles.
Open riddleVisual traps
Visual riddles
Shape, space, and perspective hide the key to these riddles. It helps to look again, try a different representation, and question what the first image seems to assume.
20 riddlesAll riddles in this topic
Five Rows
Ten trees must occupy twenty places across five rows. The escape is to let every tree work for two lines at once.
Open riddleMutilated board and dominoes
It is an elegant impossibility: it resists not because of excess combinations, but because there is something on the board that no coating can correct.
Open riddleThe 100-story building and the two eggs
It is not a problem of bravery or patience, but of worst case design. All its elegance lies in finding a plan that shines not when everything goes right, but when…
Open riddleThe boat and the ladder
A problem that invites you to calculate, but the calculation is not what matters.
Open riddleThe boat and the marbles in the lake
A boat, a lake, a handful of steel marbles. The question seems like a physics one, but the answer depends on something more subtle than simple weight. It is worth…
Open riddleThe Cake in Eight Pieces
Sometimes the mistake is not in counting badly, but in looking at a flat object when it actually has volume. A cake is not just a circle: it also has height.
Open riddleThe clock and its hands
A clock has two hands that do not rest. It seems easy to count how many times they are found, but the number that almost everyone says is not correct.
Open riddleThe Coin That Goes Around Another Coin
Intuition usually imagines that a coin going around an identical coin makes one full turn. But as it rolls, it does not only move forward: it also rotates along the…
Open riddleThe coin triangle
Sometimes a figure can be transformed completely by moving very little. Here, three coins are enough—provided you find the exact positions.
Open riddleThe double square
Like Plato's Meno, this small structure illustrates a very clear geometric concept: doubling an area does not mean doubling a side.
Open riddleThe fly between two trains
Its fame is born from a gentle trap: intuition pushes towards an endless calculation of twists and turns, but the problem has a much more direct solution.
Open riddleThe Necklace Split Fairly
Fairly dividing a mixed necklace seems to require separating beads one by one. The question is whether the continuity of the circle itself can do the work.
Open riddleThe plane and the conveyor belt
Few puzzles have generated as much debate as this one. The answer seems obvious in one sense, and it also seems obvious in the other way. It is worth stopping before…
Open riddleThe rope around the Earth
A classic problem about circles that defies intuition: the result does not depend on what one would expect.
Open riddleThe round sewers
This riddle became famous as an interview question because it seems everyday, but it forces you to reason about geometry, safety, and practical use. The main answer is…
Open riddleThe Stop at the River
Two points, a straight riverbank, and one required stop. The optimal route appears when the problem stops looking like a detour.
Open riddleThe three switches
Three switches, a light bulb in another room, and only one chance to get in to see it. It seems like there's not enough information — until you realize that the light…
Open riddleThe Wobbly Table
A wobbly table seems to call for a wedge, a folded napkin, or a different spot. But sometimes simply rotating it is enough: the irregular floor forces a balanced…
Open riddleTwo Cylinders, One Sheet
The same rectangular sheet can be turned into two very different cylinders. The amount of material does not change, but that does not mean their capacities must be equal.
Open riddle
Support the project
No ads. No algorithm. Just craft.