The note
How schools teach it now
In Year 5 children learn the primes up to 19 by heart and check whether numbers up to 100 are prime. They use 'squared' (5²) and 'cubed' (2³). A prime has exactly two factors: 1 and itself.
5-minute kitchen-table activity
Build with sugar cubes or Lego: squares (2 by 2, 3 by 3) and cubes (2 by 2 by 2). Count the pieces: 4, 9 and 8. Then circle the primes on a calendar page: 2, 3, 5, 7, 11, 13…
Worked example
Which of these is a prime number? 21, 27, 29 or 33
- 21 = 3 × 7, 27 = 3 × 9 and 33 = 3 × 11, so none of those is prime.
- 29 can't be divided by 2, 3 or 5, so its only factors are 1 and 29.
Answer: 29
Common mistakes to watch for
- Saying 1 is prime (it has only one factor).
- Thinking every odd number is prime: 9, 15 and 21 aren't.
- Confusing 3² (3 × 3 = 9) with 3 × 2 = 6.
Encouragement
- Try saying
- “Let's test it: does 2, 3, 5 or 7 go into it?”
- Rather than
- “Just guess, you'll probably get it.”