It seems very natural that if you have two dollars and add two dollars to them, you have four dollars. But do you know it too men millions of years to be able to think so? In fact, one of the most difficult things to teach children is the idea of numbers.
In ancient times, when a man wanted to tell how many animals he owned. he had no system(系统) of numbers to use. He put a stone into a bag for each animal. The more animals, the more stones he had. It may explain why the word "calculate" comes from the Latin word "calculus", "calculus" means "stone."
Then, man used lines to count. People would just draw a line for each thing they wanted to count, but they had no word to tell the number.
Later on, people used fingers. Again we have word that goes back to this. The word "digit" comes from the Latin word "digitus". It means "finger"! And the fact that we have 10 fingers led to the use of 10 in the system of numbers.
The number system we use today was invented by the Hindus in India thousands of years ago and was brought to Europe during 8th to 11th by Arab traders(商人). In this system, people count things with ten numbers. 1. 2.3.4.5.6.7.8.9.and 0. It is decimal system(十进制), that is, it is built on the base of 10.
