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Maybe it’s the chilly fall air. Or maybe it’s the macabre Halloween lawn decorations in your neighborhood. Both cool temperature and emotions can give you goosebumps. What causes goosebumps, and why can two different sources give you the same physiological reaction?

Goosebumps happen when tiny muscles at the base of each hair on your skin contract. This causes the hair to stand and the skin to buckle, making your skin look like a goose’s skin after its feathers have been plucked. The body releases the hormone adrenaline (also called epinephrine) when it feels cold, and adrenaline causes the skin muscles to contract. In animals, goosebumps fluff up the fur or feathers to trap in body heat. Humans, though, wear clothes and have a lot less hair, so this reaction doesn’t help much in keeping you warm.

Adrenaline is also released by fear or strong emotions, which is why you get goosebumps from watching a horror movie or thinking about an emotional event. In animals, this response makes their fur or quills stand up, helping the animal appear larger to scare off the attacker.

Adrenaline has a lot of other actions besides causing goosebumps. It’s a hormone released under stress, and it ramps up the body to deal with it. Other effects of adrenaline include sweaty palms, tears, trembling hands, a racing heartbeat and feeling “butterflies” in your stomach.

So this Halloween, whether it’s ghouls or just the crisp night air, know that goosebumps have you covered.

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.