Cross out the multiples. Watch the primes appear.
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This is a 2,200-year-old algorithm and still the fastest way to find all the primes below a limit by hand. Take the first uncrossed number, cross out all its multiples, move to the next uncrossed number, repeat. Whatever survives is prime.
The game walks you through it one prime at a time. You click the multiples yourself, which is the part that makes the idea stick — the primes are not found so much as left behind.
This version: The full hundred grid. Your best score is saved in this browser, so you can come back and try to beat it. Nothing is uploaded and no sign-up is needed.
A Greek mathematician and librarian at Alexandria in the third century BC. He also estimated the circumference of the Earth to within a few percent using shadows and a walking distance.
Once the current prime exceeds the square root of the limit. Every remaining number has already been crossed out by a smaller factor if it had one.
Yes. Variants of the sieve remain the standard method for generating lists of primes in computing, because nothing simpler has beaten it.