Sieve of Eratosthenes – finding prime numbers

MathematicsNumbers & ArithmeticAges 11–12

A grid of numbers from 1 to N. Each time you click 'Next step', the smallest unmarked prime is highlighted in blue and its multiples are crossed out in a matching color. Click any cell to see its factors and multiples highlighted on the grid, and find out instantly whether that number is prime or composite.