Brain picture Gray's Brain Teasers and Riddles

Please send me submissions to the following collection of brain teasers and riddles.

Marble Contest (2/24/2007)

Five youngsters entered a contest to guess how many marbles were in a bowl. Alice guessed 45, Betty guessed 39, Chuck guessed 49, Dan guessed 50, and Ed guessed 47. One was off by 6, one by 5, one by 2, and one by 4. One was right.

Bug Speed (1/31/2007)

A caterpillar crawls at a speed of 21cm per hour towards a vegetable patch. After eating its fill it returns, over exactly the same distance, at only 7cm per hour.

Not including the eating time, what is the caterpillar's average speed over the entire journey?

Gold Chains (11/09/2004)

A woman wants to buy something at an auction where you bid grams of gold instead of money. She owns a length of gold chain (with 2 ends) made of 23 interlocking loops, each weighing 1 gram. She wants to go to a jeweler before the auction to cut the minimum number of loops that would allow her to pay any sum from 1 to 23. For example, she could pay a 13 gram price with a 12 link chain and a single link. After much thought, she figures out a way to do it by cutting just 2 of the loops in the chain. How many loops are in the pieces of chains that she has after the 2 cuts?

Write in the answer as the number of loops in each chain ordered from smallest to largest, separated by a space. So if you think that 1 loop, 2 loops, 20 loops is correct then you would enter: 1 2 20. If you think that it is 1 loop, 3 loops, 5 loops, 6 loops, and 8 loops you would enter: 1 3 5 6 8.

16 Numbers (11/08/2004)

Below there are sixteen numbers. Assuming that any three of the numbers may be drawn at random, what are the chances that three numbers will be drawn whose sum equals six?

1 2 3 1
2 3 1 2
3 1 2 3
1 2 3 1

Shuffled Decks (10/22/2004)

Two identical packs of 52 cards A and B are shuffled thoroughly. One card is picked from A and shuffled with B. The top card from pack A is turned up. If this is the Queen of Hearts, what are the chances that the top card in B will be the King of Hearts?

Bad Shipping (10/14/2004)

5 guys who live in apartments make orders from the same company. Unfortunately the shipping company delivered every order to the wrong apartment.

  1. Roger, who doesn't live in an end apartment, ordered the television set.
  2. Tom lived next door to the man who received the dishware.
  3. Mr. Weiseman, who didn't receive the automotive tools, lives two apartments from the man who ordered the downhill skis, and one apartment from Harry.
  4. Ed, whose last name isn't Smith, lives in apartment #3, didn't receive the automotive tools.
  5. Mr. Smith, who doesn't live in apartment #4, ordered the golf clubs but he received the item that Mr. Campbell ordered, which wasn't downhill skis.
  6. The bachelor in apartment #1, which isn't Tom, ordered what Al received.
  7. The man in apartment #2, who didn't receive the golf clubs, lives next door to where his order was delivered.
  8. Mr. Bates didn't order the downhill skis.
  9. The television set was not delivered to Ed's apartment.
  10. Tom lives in apartment #5.
  11. Nothing is known about Mr. Harper.

More Jugs of Water (10/4/2004)

No, you cannot pour some water into the 9 liter jug and then guess. Nor can you fill each jug up half way or something. You have to be exact. A step is defined as pouring water into or out of a jug. For instance, filling the 4 liter jug and then pouring it into the 9 liter jug, and emptying the 9 liter into the pool is 3 steps.

Two Children (9/26/2004)

An acquaintance tells you she has two children and you ask her if at least one of them is a boy. She says yes. Assuming every birth has an equal chance of being male or female and you have no additional information about the children, what are the chances that both of her children are male?

100 Coins (9/13/2004)

What combination of 100 US coins add up to 500 ($5) without using nickels (5 cents). You can use silver dollars (100 cents), 50-cent pieces (50 cents), quarters (25 cents), dimes (10 cents), and pennies (1 cent). If you find one solution, can you find any more?

Write in the answer as the number of each coins as it's face value in cents ordered from smallest coin to largest, separated by a space. So if you think that 11 pennies, 2 dimes, 5 quarters, 0 50-cent pieces, and 7 silver dollars is an answer then you would enter: 11 2 5 0 7. If you think that it is 3 quarters, 4 50-cent pieces, 1 pennies, 2 dimes, and 5 silver dollars you would enter: 1 2 3 4 5.

Birthday Ring (1/8/2004)

A man is buying a gold ring set with stones for his wife on her birthday. A ring set with 4 amethysts and 1 diamond comes to US$2,000. One with 3 emeralds, 1 amethyst and 1 diamond would be US$1,400. And one set with 2 rubies and 1 diamond would cost US$3,000. Being a thoughtful husband, he choses a ring with 4 stones, each representing one of their 4 children.

As their children are named Andy, David, Ellen, and Richard, how much in US$ will the ring containing 1 amethyst, 1 diamond, 1 emerald, and 1 ruby cost him?

Pocket Change (12/4/2003)

[ Question for people who know the US coins. My apologies to any out of country visitors. ]

You and a friend pass by a standard coin operated vending machine and you decide to get a candy bar. The price is US$0.95 but after checking your pockets you only have a dollar (US$1) and the machine only takes coins.

You turn to your friend and have this conversation.

You: Hey, do you have change for a dollar?
Friend: No.
You: How about a quarter?
Friend: Yes.
Friend: Sorry, I'd like to but I can't with the coins I have.

What coins does your friend have? Write in the answer as each coins' face value in cents ordered from smallest to largest, separated by a space. So if you think the 6 coins are a dime, 4 nickels, and a quarter then you would enter: 5 5 5 5 10 25. If you think that it is a quarter, 3 pennies, a dime, and a nickel you would enter: 1 1 1 5 10 25.

Kids Ages (5/1/2003)

At a party you overhear another guest asking the age of the hosts' 3 children. The host tells the guest that the product of his kids' ages is 72 and the sum of their ages is the same as the guest's house number. Now, the other guest obviously knows her own house number but, after thinking for a bit, she is forced to ask the host for additional information. The host then says that his oldest child likes strawberry ice cream and the other guest is able to say their ages. Even though you don't know the guest's house number, you have enough information.

Assuming that all of the ages are whole numbers (1, 2, 3 not 2 1/2), How old is his oldest child?

Balloon In Car (03/31/2003)

You're sitting in a car that's not moving with a helium-filled balloon, which is resting up against the car's ceiling somewhere near its middle. The driver hits the gas and the car accelerates forward, throwing you back into your seat.

Fast Eddie (02/13/2003)

Fast Eddie the used car salesman sold a well used car for 30% more than it had cost him. The poor soul who bought the car was happy with the vehicle until it started breaking down a week later. The customer who bought the car went back to Fast Eddie and demanded he receive his money back. Eddie countered that the car had been used for a week, thus, no refund. However, Fast Eddie offered to buy the car back for 20% less than the customer paid for it. The customer agreed. Fast Eddie was then able to unload the car for the same amount that he initially paid for it.

What percentage of his costs did Eddie make on the whole deal?

Bananas to Market (02/13/2003)

A banana grower wants to transport his 3000 bananas 1000 miles across the desert to the market. All he can use is his one camel which can carry 1000 bananas at once but it needs to eat one banana to walk one mile (regardless of how many bananas it is carrying).

How many bananas can the grower get to market?

Passing Ships (2/3/2003)

You are taking a cruise Liverpool, England to New York City, USA. The passage takes an hour less than 7 days (167 hours) and then the ship is in port 23 hours before returning. The cruise line has ships which depart from NYC and Liverpool every day of the week at precisely 10:00 GMT. The ships all sail along the same route so as you make the passage, you will pass by other ships from the cruise line traveling in the opposite direction.

How many of the cruise line's ships will you pass by going in the opposite direction while making this voyage?

Finger in Glass (01/27/2003)

You have a glass of water sitting on a perfectly balanced scale. You put your finger into the glass and into the water. Your finger does not touch the glass, it only is submerged in the water. It makes the water go higher up the sides of the glass but it does not overflow.

Deep Hole (01/27/2003)

You come across a man digging a deep hole. "How deep is your hole" you ask. He replies "You'll have to guess but I can tell you that I'm 2 meters (200cm) tall". You then say "How much deeper are you going?" He answers, "I am half done. When the hole is done my head will be twice as far below ground as it is now above ground."

How deep is the current hole in centimeters?

Water in the Berries (01/27/2003)

You have a large container of berries. When the berries were fresh, 99% of their weight was from the water in the berries. A test 3 days later showed that they had lost some of the water due to drying and were now only 98% water by weight.

If the initial fresh berries weighed 100kg, how many kg do they weigh 3 days later?

Rock in Boat (8/6/2002)

You are sitting in a boat floating in a swimming pool. At the bottom of your boat is a heavy rock. You pick up the rock and drop it into water and it immediately sinks to the bottom of the pool.

Dropped Cooler (8/6/2002)

A guy is paddling a canoe up a stream at a constant speed (through the water). The water is flowing downstream at another constant speed (over ground). At some point a cooler falls out of the canoe into the water. After 10 minutes of paddling the guy notices that it is missing and turns around immediately and paddles downstream. While he is paddling downstream, he travels the same speed through the water as he did upstream. When he finally reaches the cooler, the cooler has traveled 1 kilometer from the spot where it fell out of the canoe.

How fast is the water flowing downstream (over ground) in kilometers per hour?

Bird and Trains (8/6/2002)

Two trains are traveling towards each other on separate tracks. They are each traveling at 30kph. When the trains are 1 kilometer apart, a bird leaves one of the trains and flies towards the other at a speed of 120kph. When it reaches the other train it immediately turns around and flies back the other direction. It continues to do this back and forth until the trains reach each other.

How many kilometers does the bird travel between the trains before they meet?

Clock Face (1/3/2002)

You have an analog clock whose face is number in a circle from 1 to 12, with 12 facing "north", 3 "east", 6 "south", and 9 "west". You are allowed to draw two lines which go all the way across the clock face, and divide the numbers on the clock into 3 or 4 groups depending on if the lines intersect. How can you draw the lines so that the numbers in each group add up to the same sum.

Do you want to be a wimp and have me give you the answer?

Three Advisors (6/5/2001)

A king has three advisors. He wants to find out which of them is the smartest so he devises a test. He tells them that he has 3 white hats and 2 black hats. After seating the three facing each other, he tells them that he would randomly pick 3 hats, which would be placed on their heads so that they could each see only the other two hats, but not their own. Unknown to the advisors, the king tosses away the two black hats and only uses the three white hats. After staring at each other for a while, one of the advisors figures it out and shouts, "I know! My hat is white!" How did the advisor know?


Missing Dollar (6/5/2001)

3 men go into a motel. The man behind the desk said the room is US$30, so each man paid US$10 and went to their room.

A while later the man behind the desk realized the room was only US$25, so he sent the bellboy to the 3 guys' room with US$5. On the way, the bellboy couldn't figure out how to split US$5 evenly between 3 men, so he decides to give each man US$1 and kept the other US$2 for himself.

This meant that the 3 men each paid US$9 for the room, which is a total of US$27. Add the US$2 that the bellboy kept brings the total to US$29.

Where is the other dollar!?


Crossing the Bridge (03/21/2001)

Four people are on one side of a bridge that they need to cross. It is dark, there are holes in the bridge surface, and they only have one flashlight which must travel with them as they cross. To add to their troubles, the bridge will only support two people at a time.

No matter how fast a person can cross, he must wait for his companion. If #1 goes with #2, it takes 2 minutes for them to cross. If number #2 goes with #4, it takes them 10 minutes. The flashlight cannot be thrown, instead it must be carried and delivered from person to person. The 4 people are trying to make it across in the fastest possible time to make an important appointment in the next town.

What is the fastest time in minutes for them to cross the bridge?

Jugs of Water

No, you cannot pour some water into the 5 liter container and then guess. Nor can you fill each jug up half way or something. You have to be exact. A step is defined as pouring water into or out of a jug. For instance, filling the 3 liter jug and then pouring it into the 5 liter jug, and then pouring the 5 liter jub into the pool is 3 steps.

Buried in Sand

A sadistic torturer has buried 3 men up to their necks in sand facing a wall. They cannot turn their heads. #1 is closest to the wall, then #2, and then #3. #1 sees only the wall, #2 can see #1 in front of him, and #3 sees #1 and #2. A hat, either white or black, is put on each man from behind so they cannot see what color of hat they are wearing. It is securely attached so it won't fall off under any circumstances.

Their captor then explains:

How can these men get their freedom?


Einstein's Riddle

Einstein supposedly wrote this riddle and said 98% of the world could not solve it. Found on Patrick W. Gilmore jokes page.

There are 5 houses painted in different colors in a row on a street. In each house lives a person with a different nationality. The 5 owners each drink a certain type of beverage, smoke a certain brand of cigar, and keep a certain pet. No owners have the same pet, smoke the same brand of cigar, nor drink the same beverage.


Three Switches

There are three switches in a room on the first floor and a lamp on the third floor. Each one is labeled On and Off. One (and only one) of the switches controls the lamp. There is no possible way for you to see whether the light is on or off from the first floor. You are alone in the house.


Ten Stacks of Coins


Student Ages

You overhear a conversation between a professor and his teaching assistant. They are talking about a course they are running with just three students.

Given that all of the ages involved are integers (whole numbers), how old is the professor?

Twelve Balls

So this is a fun one. I think someone sent it to me a while back and I couldn't solve it after spending a lot of time on it. Well I got it again and solved it after an hour or so. I'm sure you can do it faster.

There are (at least) two ways to solve this problem. The first way I solved it is to weigh the balls once, and then make a determination on which balls to weigh next. After I spent some more time on it, the other way to solve it is to configure all of the weighings at the start and look at the results to determine which ball is different and whether heavier or lighter.


Eight Balls


Three Hats

If the ball is black, what are the chances of you reaching back into the same hat and finding that the remaining ball in the hat is black.

The Famous Let's Make a Deal Question

What should your response be?

What are the chances of it being in the curtain you initially chose?

What are the chances of it being in the curtain to which Monty's asking you to switch?


I've moved the quicky teasers to a separate page.

More Riddles

Here are some additional puzzle sites I've come across.
