# Multiplying Cells Puzzle

A biochemist is cultivating living cells. Each cell splits into two cells after one minute. One minute later the two cells split to make four, then the four become eight, and so on. Every minute the number of cells doubles. Assume that it takes an hour for one cell to grow until a bottle is filled. If the chemist […]

# The Classic Hourglass Puzzle

Here is a classic puzzle. Say that you have two hourglasses of different sizes. The bigger hourglass runs for 11 minutes while the smaller one runs for 7 minutes. Using these two hourglasses, how can you measure 15 minutes exactly.?

# Primes That Do Not Generate Primes When Any Single Digit Is Prepended

Giving a quick inspection at the list of prime numbers, you’d notice that 2 is the first prime number. However, prepending any single decimal digit to it would not produce other primes, that is, 12, 22, 32, 42, 52, 62, 72, 82 and 92 are composite numbers. Likewise, the third prime number, 5, also shares […]

# Removing The Cherry In The Cocktail Glass: A Match Puzzle

Here is a creative match puzzle by Martin Gardner. Suppose that the four matches in the illustration above represent a cocktail glass. The objective of this puzzle is to remove the cherry from inside the glass without touching it. Also, you can only change the positions of two of the matches. The glass can either end […]

# A Little Riddle

Here’s a little riddle from Don Lemon, Everybody’s Illustrated Book of Puzzles, 1890: Twice six are eight of us, Six are but three of us, Nine are but four of us, What can we possibly be? Would you know more of us? I’ll tell you more of us. Twelve are but six of us, Five are but […]

# Full Circle

Place 24 pigs in these sties so that, no matter how many times one circles the sties, he always find that the number in each sty is closer to 10 than the number in the previous one.

# The First Odd Number In The Dictionary

Suppose that each number from 1 to 10,000,000,000 written in English and then alphabetically arranged (as in a dictionary) while ignoring spaces and hyphens. What would be the first odd number in the list?

# Curious Number Substrings

If you add all the substrings of 891, the sum is 198, which is the reverse of 891: 891 → 8 + 9 + 1 + 89 + 91 = 198 This is the only 3-digit number that has this property. There is also one 4-digit number with the same property. Can you find this […]