Number

A number is a way to count, measure, or label things. Numbers are one of the oldest and most useful ideas humans have ever invented. People use them to count sheep, measure flour, tell time, and send rockets to Mars. Almost every part of daily life depends on numbers in some way.

The simplest numbers are the counting numbers: 1, 2, 3, 4, and so on. These are also called natural numbers. People have used them for at least 40,000 years. Archaeologists have found old animal bones with rows of scratched marks, called tally marks. Early humans likely used these marks to keep track of how many days had passed or how many animals they had.

Then comes zero. Zero means "nothing here." It sounds simple, but it took humans a long time to figure out. Ancient mathematicians in India were the first to treat zero as a real number, around 1,500 years ago. The idea spread through the Arab world and finally reached Europe in the Middle Ages.

Numbers can also go below zero. These are called negative numbers, and they are written with a minus sign in front, like -5. Negative numbers are useful for things like cold temperatures or money you owe.

Not every number is a whole number. A fraction, like 1/2, sits between two whole numbers. A decimal, like 3.14, does the same thing in a different way. Together, whole numbers, fractions, and decimals can describe almost any amount you can think of.

Some numbers are special. A prime number can only be divided evenly by 1 and itself. The number 7 is prime, but 8 is not, because 8 can also be divided by 2 and 4. Mathematicians have been studying prime numbers for thousands of years and still find new things about them.

The way we write numbers today comes from India and the Arab world. The symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 are called Arabic numerals. They reached Europe around 800 years ago and slowly replaced Roman numerals like IV and XII. Arabic numerals were faster to write and made math much easier.

Numbers never run out. You can always add one more. This idea is called infinity, and it means a number larger than any number you could ever name. No matter how high you count, infinity is still further on.