There are all kinds of numbers: even, odd, positive, negative, and prime numbers, just to name a few. Did you know that there are even twin prime numbers? In this lesson, you’ll learn what twin prime numbers are and some interesting facts about them.

## Introducing Twin Primes

Can you answer this riddle? There were 2 babies born on the same day, in the same hospital, by the same parents, but they were not twins. How did this happen? The answer may surprise you: They were 2 babies from a set of triplets!

You know what a twin is, but do you know what a twin prime number is? In this lesson, we’re going to explore twin prime numbers and learn how to identify them. But first, let’s quickly review prime numbers.

## Prime Numbers

A **prime number** is a whole number greater than 1 that only has whole number factors of 1 and itself. When we say that it only has whole number factors of 1 and itself, we mean the only two whole numbers you can multiply to get that number are 1 and the number itself.

For example, if you want to multiply two numbers to get the number 7, the only way to do it is 7 x 1 = 7. Another example of a prime number is 5. The only way to multiply whole numbers to get 5 is 5 x 1 = 5. There’s no other way.

The first 9 prime numbers are 2 , 3 , 5 , 7, 11, 13, 17, 19, and 23.

If you do the math, you’ll see that the only factors of these prime numbers are 1 and the number itself.

## Twin Prime Numbers

Now that we’ve refreshed our memory of prime numbers, let’s dig into twin prime numbers. **Twin prime numbers** are two consecutive prime numbers that differ by 2. This is easy to remember when you remember that twins are a set of 2, and twin primes differ by 2.

For example, from our set of 9 prime numbers, there are 4 sets of twin prime numbers: 3 and 5, 5 and 7, 11 and 13, and 17 and 19. The numbers in each of these pairs differs by exactly 2.

## Practice

Now let’s practice finding the twin prime numbers from a list of the remaining prime numbers up to 100. Read through these prime numbers and identify the twin prime numbers:

Did you find them? There are 4 sets of twin prime numbers here:

- 29 and 31
- 41 and 43
- 59 and 61
- 71 and 73

Keep practicing locating twin prime numbers on your own to become a twin primes master!

## Fun Facts

Did you know that all twin prime numbers – except 3 and 5 – have a multiple of 6 between them? Look… 5 and 7 are twin primes, and the number 6 sits between them. If you look at the twin primes 17 and 19, you’ll see that 18 sits between them and 18 is a multiple of 6. Pretty neat, right?

Another interesting thing about twin primes is that some mathematicians believe that twin prime numbers go on forever. This is just a theory, though – it hasn’t been proven. But mathematicians have found some pretty big twin primes. The **largest set of twin primes** we know of so far was discovered in 2016, and it has more than 388,000 digits in the pair! Those are some huge numbers!

## Lesson Summary

A **prime number** is a whole number greater than 1 that only has two whole number factors: 1 and itself. A **twin prime number** is a pair of consecutive prime numbers that differ by two.