Speaker 1: Streaming TV shows and movies directly to your home is
Speaker 1: a breeze with Netflix. As a Netflix member, you can
Speaker 1: instantly watch TV and movies on your PC, Mac, mobile device,
Speaker 1: or television. Get a free thirty day trial membership. Go
Speaker 1: to Netflix dot com slash stuff and sign up today.
Speaker 1: Welcome to brain Stuff from how Stuff works dot com
Speaker 1: where smart happens. Hi. I'm Marshall Brain with today's question.
Speaker 1: Why does the hair on your arms stay short while
Speaker 1: the hair on your head can grow very long? Each
Speaker 1: hair on your body grows from its own individual hair follicle.
Speaker 1: Inside the follicle, new hair cells form at the root
Speaker 1: of the hair shaft. As the cells form, they push
Speaker 1: older cells out of the follicle. As they're pushed out,
Speaker 1: the cells die and they become the hair that we see.
Speaker 1: A follicle will produce new cells for a certain period
Speaker 1: of time, depending on where it's located on your body.
Speaker 1: This period is called the growth phase for the hair.
Speaker 1: Then it will stop growing for a period of time
Speaker 1: This is called the rest phase, and then start the
Speaker 1: growth phase again. When a hair follicle enters the rest phase,
Speaker 1: the hair shaft breaks off, so the existing hair falls
Speaker 1: out and a new hair takes its place. Therefore, the
Speaker 1: length of time that a hair is able to spend
Speaker 1: growing during the growth phase controls the maximum length of
Speaker 1: that hair. The cells that make the hairs on your
Speaker 1: arms are programmed to stop growing every couple of months,
Speaker 1: so the hair on your arms stays short. The hair
Speaker 1: follicles on your head, on the other hand, are programmed
Speaker 1: to let the hair grow for years at a time,
Speaker 1: so the hair can grow very long. Animals that shed
Speaker 1: have hair follicles that synchronize their rest phase, so all
Speaker 1: the follicles enter the rest phase at once. This way,
Speaker 1: all the hair falls out at the same time. A
Speaker 1: dog that sheds will lose its hair in large clumps.
Speaker 1: Some animals can also switch the coloring agent in the
Speaker 1: hair follicle audit off, so in the summer of the
Speaker 1: hair is pigmented brown with melanin, but in the winter
Speaker 1: it's not pigmented, leaving the hair white. For more on
Speaker 1: this and thousands of other topics, because it how staff
Speaker 1: works dot com. Audible dot com is the leading provider
Speaker 1: of downloadable digital audio books and spoken word entertainment. Audible
Speaker 1: has over one hundred thousand titles to choose from to
Speaker 1: be downloaded to your iPod or MP three player. Go
Speaker 1: to audible podcast dot com slash brain stuff to get
Speaker 1: a free audio book download of your choice when you
Speaker 1: sign up today