I am actually curious to this measurements thing. How do you know that a certain distance of 1 cm is 1 cm? I will be a bit clearer, we all know how long is 1 cm right but it strike me to ask how did people know that the certain length or distance is 1 cm? Who came up with these measurements idea? This concept applies to every other units such as m,km,nm and all those units. How do you know for sure 100 cm is 1m?