It’s late at night, and you’re home alone. All of a sudden, a sound—the creak of a door being opened—startles you. You look down at your arm and see goose bumps. This video from NPR answers our most-pressing questions about the bizarre bodily reaction. Turns out, goosebumps are part of the adrenaline response sparked by fight or flight situations—tiny muscles contract, raising the hair on your arms so you appear more intimidating (and stay warm in the cold).

Goose bumps can also happen in moments that provoke intense emotion, moving us so much our brain thinks there’s an emergency. File this away for when you’re looking at pictures of your friend’s baby—whether that means you’re scared, moved, or just standing under a vent.