We use 12 a lot because of lunar cycles (roughly 12 months in a year), and because it has a lot of divisors that make it mathematically useful (2, 3, 4, 6). Ten only factors into 2 and 5; it's actually a pretty bad number to choose as a numerical base. But we use it anyway, and it's because we have ten fingers and thus have a natural inclination to count in tens (which is not to say that we're

*incapable* of doing otherwise; some cultures have had base-60 or base-20 mathematics, but those are still multiples of ten).