Party flash goes off and your eyes turn into glowing red demons. It’s not the camera inventing the color—it’s lighting up the blood vessels that have been at the back of your eye the whole time. Here’s the quick biology of the classic red-eye effect and the simple ways to stop it.

Why do your eyes glow an unnatural red in flash photos? The red-eye effect happens when the camera flash lights up the blood-rich choroid at the back of your eye before your pupils can close.

In dim light your pupils open wide. The flash is faster than your iris, so light races in, reflects off the choroid’s blood vessels, and bounces straight back into the camera lens. Less melanin means more bounce. Kids get it more often. Animals get green or yellow eyeshine from a different mirror layer called the tapetum lucidum—humans never got that upgrade.

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