Eratosthenes' sieve

Eratosthenes was a Greek astronomer who lived around 200 Bc. He calculated the circumference of the earth from calculations based on the sun shining down a well.
Eratosthene's sieve is a way of producing the prime numbers.

Starting with a hundred square, you cross out multiples of 2, but not 2, then the same with 3, 5, 7, 11 and so on. The numbers not crossed out are the prime numbers.
It was tricky to read all the numbers to 100, so we did the excercise again, this time just up to 30.