### 1. If you write out pi to two decimal places, backwards it spells "pie".

### 2. A French word for pie chart is "camembert".

### 3. The spiral shapes of sunflowers follow a Fibonacci sequence.

### 4. The Fibonacci sequence is encoded in the number 1/89.

### 5. A pizza that has radius "z" and height "a" has volume Pi × z × z × a.

### 6. The word hundred is derived from the word "hundrath", which actually means 120 and not 100.

### 7. 111,111,111 × 111,111,111 = 12,345,678,987,654,321.

### 8. In a room of just 23 people there’s a 50% chance that two people have the same birthday.

### 9. Zero is the only number that can't be represented in Roman numerals.

### 10. (6 × 9) + (6 + 9) = 69.

### 11. We tend to think of odd numbers as male and even numbers as female.

### 12. If you shuffle a pack of cards properly, chances are that exact order has never been seen before in the whole history of the universe.

### 13. Zero is an even number.

### 14. There's not enough space in the known universe to write out a googolplex on paper.

According to Carl Sagan in the original Cosmos series. A googolplex is 10 to the power of a googol, or 10 to the power of 10 to the power of 100. This website will write it out for you (or start... it won't ever finish because your computer won't have enough memory).

### 15. The most popular favourite number is 7.

Nearly 3000 people, around 10% of the total asked, chose 7 as their favourite number in an online poll by Alex Bellos. The second most popular was 3.

### 16. That might be because 7 is "arithmetically unique".

It's the only number below 10 you can't multiply or divide and keep within group. For example, 5 you can multiply by 2 to get 10 (still within the 1-10 group), 6 and 8 you can divide by 2.

### 17. 7 also shows up a lot in human culture.

We have seven deadly sins, and seven wonders of the world. Not to mention colours of the rainbow, pillars of wisdom, seas, dwarves, days in the week...

This might be because when these things came about there were celestial bodies visible in the sky (the Sun, the Moon, Venus, Mercury, Mars, Jupiter and Saturn).

### 18. The number 4 is considered unlucky in much of Asia.

That's because the words for "four" in Japanese, Cantonese, Mandarin and Korean (shi, sei, si, sa) sound the same as the words in those languages for death.

### 19. .999999... = 1

Here's the proof:

If 10N = 9.9999...

Then N = .9999....

Subtract N from 10N, leaving you with 9N=9.

So then N=1. But we already know that N=.9999... as well.

So 1=.9999....

### 20. Cicadas use prime numbers as an evolutionary strategy.

Cicadas incubate underground for long periods of time – 13 or 17 years – before coming out to mate. 13 and 17 are both prime numbers. It's thought cicadas ended up in these prime number life-cycles because it meant they came into contact with predators on more round numbered life-cycles less often.

### 21. 10! seconds is exactly 6 weeks.

10! means 10 factorial. 10! = 10 × 9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 = 3628800 seconds. Which is 42 days, or 6 weeks, exactly.

### 22. Take any four digit number, follow these steps, and you'll end up with 6174.

1. Choose a four digit number (the only condition is that it has at least two different digits).

2. Arrange the digits of the four digit number in descending then ascending order.

3. Subtract the smaller number from the bigger one.

4. Repeat.

Eventually you'll end up at 6174, which is known as Kaprekar's constant. If you then repeat the process you'll just keep getting 6174 over and over again.