Primes

knock out the multiples. see who survives.

the sieve of Eratosthenes under your finger

A prime number is defined by what it isn’t: nobody’s multiple but its own. Clearing out the multiples of a few small numbers is enough to prove it — once 2, 3, 5, and 7 have knocked out everything they touch, whatever’s left standing up to 100 is prime, without ever checking a single number for divisibility by hand.

Concepts: primes, number-theory.

An interactive you play with one thumb, on Wundr.