Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
Human brain
As a hominid brain, the human brain is substantially enlarged even in comparison to the brain of a typical monkey. The sequence of human evolution from "Australopithecus" (four million years ago) to "Homo sapiens" (modern humans) was marked by a steady increase in brain size. As brain size increased, this altered the size and shape of the skull, from about 600 cm in "Homo habilis" to an average of about 1520 cm in "Homo neanderthalensis". Differences in DNA, gene expression, and gene–environment interactions help explain the differences between the function of the human brain and other primates.