Most people use base ten for counting, likely because humans have ten fingers on their two hands.

The longer version

Anatomy is the obvious suspect: ten fingers are always available, require no equipment, and give a natural stopping point for tallying. Once a collection is grouped into tens and tens of tens, the vocabulary can stop inventing new words and start combining old ones, which is exactly how English builds twenty-nine or sixty-four.

The pattern is not universal, which weakens any single cause. Some cultures counted in twenties, adding toes to fingers, and vestiges survive in phrases built on twenties rather than tens. Twelve also left deep marks in units of length and weight. Ten probably won because it was good enough, taught easily, and spread with the traders who used it.

Where this entry sits

This section collects odd corners of mathematics, from stubborn prime numbers to counting systems older than modern schools. Expect famous constants, probability puzzles, shapes that break everyday intuition, and numbers far too large to picture. The full list sits on the Numbers and Math page, where every entry is listed in order.