Cross Out Multiples of 2, of 3 — What's Left Is Only Primes!
Lay out the numbers from 2 to N, and try clicking a prime number yourself. Click 2, for example, and every multiple of 2 — 4, 6, 8, … — gets crossed out at once; click 3 next, and every multiple of 3 — 6, 9, 12, … — gets crossed out too. Crossing out numbers this way, one after another, until only the survivors remain, is called the Sieve of Eratosthenes. You can also step through it in order with the "Next Step" or "Auto Play" buttons.
How it works
Lay out every number from 2 to N. At the start, all of them are "unknown."
Among the numbers not yet crossed out, the smallest one is always prime — it can't be a multiple of any smaller number.
Click that number (or press "Next Step"), and every multiple of it (×2, ×3, ×4, …) gets marked as "crossed out as a multiple."
Repeat this all the way up to N, and the numbers that never get crossed out are exactly the primes.
Range (2 ~ N)100
Unknown (click it)Confirmed primeCrossed out as a multiple
Primes found so far
0
Click a number, or press "Next Step," to start exploring primes!