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Speaker 1: Welcome to brain Stuff from how Stuff works dot com

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Speaker 1: where smart happens. Hi. I'm Marshall Brain with today's question.

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Speaker 1: Why does the hair on your arms stay short while

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Speaker 1: the hair on your head can grow very long? Each

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Speaker 1: hair on your body grows from its own individual hair follicle.

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Speaker 1: Inside the follicle, new hair cells form at the root

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Speaker 1: of the hair shaft. As the cells form, they push

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Speaker 1: older cells out of the follicle. As they're pushed out,

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Speaker 1: the cells die and they become the hair that we see.

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Speaker 1: A follicle will produce new cells for a certain period

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Speaker 1: of time, depending on where it's located on your body.

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Speaker 1: This period is called the growth phase for the hair.

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Speaker 1: Then it will stop growing for a period of time

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Speaker 1: This is called the rest phase, and then start the

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Speaker 1: growth phase again. When a hair follicle enters the rest phase,

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Speaker 1: the hair shaft breaks off, so the existing hair falls

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Speaker 1: out and a new hair takes its place. Therefore, the

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Speaker 1: length of time that a hair is able to spend

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Speaker 1: growing during the growth phase controls the maximum length of

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Speaker 1: that hair. The cells that make the hairs on your

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Speaker 1: arms are programmed to stop growing every couple of months,

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Speaker 1: so the hair on your arms stays short. The hair

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Speaker 1: follicles on your head, on the other hand, are programmed

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Speaker 1: to let the hair grow for years at a time,

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Speaker 1: so the hair can grow very long. Animals that shed

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Speaker 1: have hair follicles that synchronize their rest phase, so all

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Speaker 1: the follicles enter the rest phase at once. This way,

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Speaker 1: all the hair falls out at the same time. A

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Speaker 1: dog that sheds will lose its hair in large clumps.

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Speaker 1: Some animals can also switch the coloring agent in the

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Speaker 1: hair follicle audit off, so in the summer of the

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Speaker 1: hair is pigmented brown with melanin, but in the winter

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Speaker 1: it's not pigmented, leaving the hair white. For more on

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Speaker 1: this and thousands of other topics, because it how staff

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