Why do we count things in group of ten? The reason (原因) is that we have ten fingers. Long 1, when people had to count a lot of things, they matched them against their 2 . First they counted out enough objects to match the fingers of 3 and then put them 4 in one group. 5 there were more than ten, they formed more groups. We can call our numbers "two-handed" numbers, because they developed from the 6 that people counted things on two 7.
Some people had "8 numbers", too. Because there are five fingers on one hand, they counted things out in groups of five. It was 9 by Romans (罗马人) who lived in Italy over 2000 years ago. We call their 10 numbers Roman numerals and we use them even today. In Roman numerals, "Ⅰ" stands for one, and "V" stands for five. 11 six, the Romans wrote "VI".
12 people counted objects in groups of 12. We 13 use the 12-in-one-group system(系统) for telling time with a clock. We count the 14 from one to twelve and then start 15 one again.
