Prime numbers are those numbers (greater than 1) that cannot be divided by any number except themselves and one.

The Greek Eratosthenes created a method to find out these prime numbers, although it only worked over a limited range:

1) Write out the numbers from 1 to 100 in ten rows of 10.

2) Cross off number 1, because all primes are greater than 1.

3) Number 2 is a prime, so we can keep it, but we need to cross off the multiples of 2 (i.e. even numbers).

4) Number 3 is also a prime, so again we keep it and cross off the multiples of 3.

5) The next number left is 5 (because four has been crossed off), so we keep it and cross of the multiples of this number.

6) The final number left in the first row is number 7, so cross off its multiples.

7) You have finished. All of the "surviving" numbers (coloured in white below) on your grid are prime numbers.

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