# Prime numbers

How many prime numbers are there, and why do some people call them the building blocks of numbers?

A prime number is a whole number that cannot be divided exactly by any other numbers except 1 and itself. Most of us are familiar with the starting sequence – 2, 3, 5, 7, 11. But did you know that there are infinitely many prime numbers?

Primes aren't just a mathematical curiosity, though. For example, you can make any whole number bigger than 1 by multiplying primes together - and for each number, there's exactly one way to do it. So in a sense, you can think of primes as the building blocks of all other numbers. The primes you multiply together to make a number are called its 'prime factors'.

More practically, primes can used to make hard-to-break codes, like the ones used to keep credit card numbers safe when buying online. One popular code, RSA, relies on the fact that it's hard to work out the prime factors of a big number, but easy to get a big number that you already know the prime factors of - you just work out two big primes and multiply them together.

There are plenty of mysteries about prime numbers, too. For example, the 18th century mathematician Christian Goldbach suggested that every even number greater than 2 could be made by adding together two prime numbers. This has been checked for every even number up to 5,400,000,000,000,000,000, but nobody has managed to prove there isn't a bigger number that doesn't follow the rule - despite the \$1,000,000 prize that was offered from the year 2000 to 2002.