
Which Bicycle Wheel Turns Faster
Which bicycle wheel makes more turns over a given path — front or rear? Assume both wheels have the same diameter.

The Last Knight
White to move.

Risk and Dunbar Number
If you personally know somebody who died from X, then X is dangerous.
This risk assessment heuristic is not as stupid as it sounds.

Your Friends Have More Friends than You Do
Henry Heather   Helen — Cindy — Carlos — Olivia — Harold   Harry Hector 
Two Envelopes
You are given two envelopes. Each envelope contains some money. You can pick one envelope, count the money inside it, then either keep this envelope, or return it and keep the other one.
Can you do better than just picking an envelope at random and keeping it?

Stick+Pencil
When would somebody hold the stick like shown, and why?

Timelapse of Tappan Zee Bridge

Where to Wait for an Elevator
You are waiting for an elevator. There are four elevators, equally spaced along a wall. The third elevator is out of order. Where would you stand, and why?

Determining Cat Chirality
Does your cat prefer to sleep like this ⟳ (clockwise), like this ⟲ (counterclockwise), or has no preference? Let’s figure it out:
 Print this chart
 Put a pin or a dot in the lower left corner (at coordinates (0,0))
 When you observe ⟳, move the pin right by one
 When you observe ⟲, move the pin up by one
 If the pin reaches the red line, stop the test. Cat prefers ⟲
 If the pin reaches the green line, stop the test. Cat prefers ⟳
 If the pin reaches the yellow line, stop the test. No preference detected

Raymond Smullyan, 1957
The white king has just been knocked off the board. Where was it standing?

Everyday Carry: Car
Contents of the small backpack I keep in the trunk and take with me when hiking.

CarlsenKarjakin 2016, Game 16
White to move.

Periscope

Is 333,333,333,333,333,331 Prime?
31 is prime. 331 is prime. 3,331 is prime. 33,331 is prime. 333,331 is prime. 3,333,331 is prime. 33,333,331 is prime. Is 333,333,333,333,333,331 prime?

Dragon Curve
Take a strip of paper and fold it in half to the right. Then fold it in half again, in the same direction. Repeat several times, then open out so that each fold becomes a 90 degree turn. This example uses paracord instead of paper, but idea is the same.

Estimating Blast Yield
When photos of the first atomic bomb test were published, its blast yield was still classified. But there is enough information in this picture to determine it.

A Sin to Err
A toy pen and paper cipher

Right Triangle Area
Find the area of the right triangle with hypotenuse 10 and height (dropped from the right angle) 6.
This problem was on a math test for quite some time, until it turned out that the best students (and only the best students) could not solve it.

Trucker's Hitch

Irrational Exponentiation
What is greater, or ?

Four Moves
This is the position after black move 4. What were the moves?

Rolleron
Back when electronic components were heavy and expensive, this nifty contraption was used to prevent airtoair missiles from spinning en route to their target.

Switch from macOS to Linux
(E)migration from macOS to Linux is complete.
Left: early 2013 13 inch MacBook Pro Retina with 2.6 GHz Intel Core i5 running macOS 10.12.4.
Right: early 2017 Dell XPS 13 with 2.7 GHz Intel Core i77500 running Ubuntu 16.04.

Square from Four Points
You are given one point from each side of a square. Reconstruct the square with compass and straightedge.

Adjustable Grip Hitch

Richard Réti, 1921
White to move.

Double Fisherman's Double Loop

Not Quite Random
Let’s say we want to estimate with Monte Carlo. We get some pairs of random numbers with both and uniformly distributed between 0 and 1. Approximately of those should fall within the unit circle, so we can get an estimate of by multiplying proportion of points that fall within that circle by 4.
Knowing that some implementations of pseudorandom number generators are better than others, we decide to take a look at the first 256 points. Can you tell which of the two pictures below shows uniformly distributed random numbers?

Fluorinert

Poacher's Knot

CarlsenKarjakin 2016, Game 8
Black to move.

Comparing Logs
What is greater, or ?

Aerial Photography
An airplane is taking pictures of the ground. Its camera is facing straight down. The camera is loaded with a long roll of film that is slowly advanced by a motor.
Come up with a system that controls the rate of advance so that the film moves at the same speed as the image projected by the lens, regardless of airplane speed and altitude.
Complication: only use technology available in 1950’s. Photosensors and basic electronics is ok, but no CCD and no computers.

Leonid Kubbel, 1922
White to move.

Kalmyk Loop

Estimating BDBA Probability
DBA (Design Basis Accident) is an accident that a nuclear facility is designed to withstand.
BDBA (Beyond Design Basis Accident) is an accident that was not fully considered in the design process because it was judged to be too unlikely.
Let’s try to quantify how unlikely.

Richard Réti, 1922
White to move.

Calculating 52! by Hand
It’s easy  calculate 54!, then divide by 53 and 54.

Under Construction
In 2017 I plan to publish here one blog post per week, starting on January 1st.
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