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!

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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.