A natural number is any of the counting numbers you use to count things: 1, 2, 3, 4, 5, and so on forever. When you count your friends, your pencils, or the days until your birthday, you're using natural numbers.

These numbers are called “natural” because humans started using them naturally, long before anyone invented writing or formal mathematics. Ancient people counted their sheep, their harvests, and their trading goods using these simple numbers. You can't have half a sheep or negative three apples when you're counting real objects, which is why natural numbers are always whole and positive.

Mathematicians sometimes disagree about whether zero counts as a natural number. Most treat the natural numbers as starting at 1, but some definitions include 0 because it represents counting nothing, which is a useful concept in mathematics.

Natural numbers are the foundation for all other types of numbers. Once you understand natural numbers, you can build up to integers (which include negative numbers), fractions, decimals, and beyond. But natural numbers remain the most basic and intuitive, the ones you probably learned before you even started school, when you first counted on your fingers or counted your toys.