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As we all know, cold weather will cause us to have goose bumps. I honestly don't know why (1), because our skin contracts?

Apart from that something quite contradicting happened this morning. I was taking a hot shower and it was so hot that I suddenly felt that I had goose bumps. Not because it was cold, but because it was so hot. How can this happen? Because of a sudden change in temperature the skin goes haywire? Or aren't goose bumps related to temperature in a way, but is a sort of safety reflex?

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Goose Bumps (a.k.a. - Goose Pimples) are caused by the Arrector Pili muscles in your skin:

Skin diagram

Their purpose is to increase the distance from epidermis to the top of the follicle, and increase the area between follicles. This has the effect of increasing the amount of air trapped between the hair and surface of the skin. In other mammals the end-result is an increase in the insulatory properties of their fur, meaning they will stay warmer or cooler depending on their current needs.

Another function in our mammalian cousins is to simply appear larger, and thus more of a threat to any predators or dangerous animals. Much like the shoulder and lower back hair on this dog:

Dog snarling

Thermoregulation in humans has evolved since our furrier ancestors; we sweat to cool off, and shiver to keep warm. We also use tools and weapons, can create fire, and have other ways to communicate our lethal capabilities.

Goose Bumps are the human body utilizing old instinctual reflexes in the proper situations, but we lack the thick mattes of hair our ancestors had for the reflex to have any significant effect. They're a fun Easter-egg, a leftover that doesn't do us any harm and are an easy way to tell if someone is cold or scared - which might be useful under the proper circumstances.

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Great answer! Will wait for a day or two before accepting. Give other peeps a chance as well. :) –  Bram Vanroy Mar 19 '13 at 17:06
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