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Riddles


Egg Weighing
Difficulty: Hard

You have 12 eggs in total, one of which is a different weight than the rest, and a two-sided balance. You are allowed to use the balance up to 3 times, after which you must tell me which egg is the odd one out, and whether it is heavier or lighter than the other 11.

Egg Sorting
Difficulty: Easy

You have 100 eggs in total, whose average weight is 1 ounce. You also have a sorting machine which will organize your eggs from lightest to heaviest. How many eggs at most can you select out of the 100 that you can guarantee to be under 2 ounces?

TRIVIA: The logic involved in this riddle is also present in Claude Shannon's famous proof that you can achieve reliable communication over a noisy channel in his seminal paper "A Mathematical Theory of Communication".

Colored Hats
Difficulty: Easy

There are five hats in a room, three of which are white, and two are red. Three men close their eyes and choose a random hat to wear. The remaining two hats are taken away. No man can look at his own hat. He can only look at the hats of the other two men. The first man is asked "what's the color of your hat"? He looks at the other two men, and responds "I don't know". The second man is asked the same question, and he responds "I don't know" as well. What color hat is the third man wearing?

Colored Eyes
Difficulty: Very Hard

In a world of only blue and green eyed people, their God hosts a competition every year. Every year, their God plucks a hundred people, ensuring at least one person of each eye color, and places them on an island. The people may not communicate with each other in any way. At midnight, every day, the people who can logically deduce their own eye color go to heaven. The rest remain. For each night of the year, how many people leave for heaven?

BONUS: Why is it important that I tell you this competition happens every year? What if I told you that one day God simply placed 100 people on an island, and looking around they can see that there's a mix of blue and green eyed people? Is the riddle still solvable?

Assholes on Planes
Difficulty: Either Very Hard or Very Easy

298 people, 1 asshole, and you, are waiting to board a plane that seats 300 people. The asshole sits at whatever random seat he wishes. Everyone else sits in their own seat if unoccupied, otherwise they sit at another random seat. You are the last person to board the plane. What are the chances that your seat is occupied?

Academic Parties
Difficulty: Medium

A professor and his wife is hosting a house party and invites four other couples. Not everyone knows each other, so the professor begins the night by asking everyone to shake hands with the people they do not know. The professor then asks the nine people (eight guests and his wife) how many hands they ended up shaking. He gets nine different answers. How many hands did his wife shake?

CLARIFICATION: You may assume that husbands know their wives and vice versa, and that if person A knows person B, then B will also know A.

Elves and Lightswitches
Difficulty: Easy

In the room, there are a hundred lights, labeled from 1 to 100, each controlled by a lightswitch. Currently all the lights are off. There are also a hundred elves about to play with the lightswitches. The first elf goes and toggles every lightswitch. The second elf then goes and toggles every second lightswitch (ie. 2, 4, 6, ..., 100). The third elf then toggles every third switch (ie. 3, 6, 9, ..., 99). And so on and so forth. After every elf has gone, which lights are on and which are off?

Physics Questions


Treadmill
Why is it more tiring to run at the same speed on a flat track in a gymnasium than on a treadmill?

Two Asteroids
There are two asteroids floating by themselves in space. The first asteroid is stationary, and the second asteroid is revolving around the first one. You are standing on one of them. How can you tell which asteroid you are standing on?

BONUS: For those that think they have the answer, here's something to think about. What do we actually mean when we say "the Earth revolves around the Sun"?

Aircraft Carrier
When a F22 fighter jet traveling at 20 meters per second lands, it takes 20 meters after touching the ground and applying the brakes to come to a complete stop. But the same jet traveling at the same speed is able to land on an aircraft carrier with only 15 meters of runway. Explain how this can be so.

Coffee and Cream
You have a cup of hot coffee to which you will add room-temperature cream. You are currently occupied so you will have to drink your coffee a few minutes later, but you would like for the coffee to be as hot as possible when you drink it. Should you add the cream now or should you add the cream just before you drink it? Why?

3D Glasses
3D glasses are typically made using polarizing filters. The left eye polarizer is polarized in a different direction than the right eye polarizer. While wearing a pair of 3D glasses, you close one eye, and look at your reflection in the mirror. You notice that the lens over the closed eye appears black, and the lens over the open eye appears clear. Why?

3D Glasses 2
While wearing a new pair of 3D glasses (different from the pair in 3D Glasses 1), you also close one eye, and look at your reflection in the mirror. You notice, this time, that the lens over the open eye appears black, and the lens over the closed eye appears clear. Why?

Car Rides
You are in your car waiting at a red light at the intersection on your way to a birthday party. Floating against the roof of your car is a helium balloon. When the light turns green, you start accelerating the car. Describe the motion of the balloon as the car accelerates.