Most numbers are composite numbers. Composite numbers have more than two factors, meaning they can be divided by more than two numbers. However, a few numbers are prime numbers. A prime number can only be divided evenly by 1 or itself.
- 5 is a prime number because it can only be divided exactly by 1 or 5.
- 6 is not a prime number because it can be divided by 1, 2, 3, and 6.
Prime numbers are one of the toughest concepts younger children will face, so teaching them how to pick out prime numbers can be tough. Luckily enough, there’s a relatively simple way of finding prime numbers, and it’s called the Sieve of Eratosthenes.
Firstly, buy a number chart that goes up to 100. Next, teach your kids the following steps to mark off all the composite numbers and leave the prime numbers:
- Cross off all multiples of 2 that are greater than 2 (4, 6, 8, 10, and so on)
- Cross off all multiples of 3 that are greater than 3 (6, 9, 12, 15, and so on)
- Cross off all multiples of 5 that are greater than 5 (10, 15, 20, 25, and so)
- Cross off all multiples of 7 that are greater than 7 (14, 21, 28, 35, and so on)
If the table goes above 100, you should also ask them to strike out all multiples of 11 and 13. When you’re done, you should be left with only the prime numbers.
The great thing about the Sieve of Eratosthenes is that it makes learning prime numbers into a game, so your children will quickly learn which numbers are always left over when the game is over. It’s also an exercise that encourages them to use what they’ve learnt about multiples, and you can use the Sieve of Eratosthenes to create helpful teaching resources that can be hung up around your home. That should be particularly beneficial if you home-school your children.