Primes & Composites
A prime has exactly two factors: 1 and itself
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A number with exactly two factors — 1 and itself — is prime; more than two makes it composite; and 1 is the odd one out, neither. Tap numbers on the chart to count their factors, then tap “Sieve” to run Eratosthenes’ two-thousand-year-old method: cross out multiples of 2, 3 and 5, and only primes remain.
Tap numbers on the chart to see their factors: exactly two makes a prime, more makes a composite, and 1 is the special one. Tap “Sieve” to run Eratosthenes' sieve — cross out multiples of 2, 3 and 5, and only primes remain.