Byron works in a lumber yard. His boss just cut a dozen planks and asked Byron to double check that they are all the same length, The planks were numbered 1 through 12. Byron took out plane number 1 and checked that the other planks are all the same length as plank 1. He concluded that they must all be the same length. Explain how you known plank 7 and plank 10 are the same length even though they were never directly compared to each other?