Step 1 of 4 · Quick recap
A prime has exactly two factors: 1 and itself. Everything else (except 1) is composite.
First few primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, ...
(2 is the only even prime — every other even number has 2 as a factor.)
Step 2 of 4 · Sieve of Eratosthenes
Use the sieve: circle each smallest remaining number as prime, then cross out all its multiples. Whatever's left is prime.
Click "Play" to watch the sieve in action.
prime
composite
neither (1)
Step 3 of 4 · Tap the primes 1–50
Tap each number that is prime. Greens stay, reds bounce back.
Found 0 of 15 primes
Step 4 of 4 · Count the primes
How many primes from 1 to 20?
Pick the count.
🏆
Nice work!
You found the primes.