Ten is the number that comes after nine and before eleven, written as 10. It's the foundation of our entire number system because we count in groups of ten: when you reach ten ones, you get one ten; when you reach ten tens, you get one hundred; when you reach ten hundreds, you get one thousand. This pattern continues forever.

We use a base-ten system (also called decimal) likely because humans have ten fingers, making it natural to count in groups of ten. Other civilizations experimented with different systems: the ancient Babylonians used base-sixty, which is why we still have sixty seconds in a minute and sixty minutes in an hour.

The number ten appears throughout our lives in important ways. A decade is ten years. The metric system organizes everything in tens: ten millimeters in a centimeter, ten centimeters in a decimeter, ten decimeters in a meter. In many sports and games, ten represents perfection or completion: a perfect gymnastics score is a ten, and bowling uses ten pins.

When someone counts to ten before responding to something that upset them, they're using those ten seconds to calm down and think clearly. The expression “a perfect ten” means something completely excellent or ideal.