Most numbers can be broken down into other smaller whole numbers, or ‘factors’ (see here). For example, 4 can be broken down into two lots of 2.

However, some numbers cannot be broken down in this way, and can only be divided exactly by themselves or by 1. For example, there are no two numbers that 13 can be divided by, except for 1 and 13. Unbreakable numbers like these are called ‘prime numbers’. The first few are 2, 3, 5, 7, 11 and 13, but there are other much higher prime numbers that, surprisingly, can still only be divided by themselves or by 1.

Did you Know?

Mathematicians love prime numbers and, although they are easy to understand, they are very mysterious. This is because they don’t fit into a regular pattern, so there is no way to find them except by trial and error.

Some mathematicians compete to see who can find the next highest prime number. The most recent one to be found has more than 12.9 million digits. It’s so long that it would take more than two months to write out by hand!