Even & Odd

House numbers on one side of a street run 2, 4, 6, 8… and on the other, 1, 3, 5, 7…. This alternation — even and odd — is the oldest number pattern humans noticed. But parity isn't just a label: it's a property you can compute with. Add two odds and you always get an even; multiply anything by an even and the result is even. These rules are the gateway to modular arithmetic.