Primes

knock out the multiples. see who survives.

the sieve of Eratosthenes under your finger

A prime is defined by what it isn't: nobody's multiple but its own. Clear the multiples of 2, 3, 5, and 7 and 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.