A flashlight beam at night makes a cat's eyes shine.
Behind the retina, the light-sensing layer at the back of the eye, cats have a mirror-like layer, the tapetum lucidum. Light that slips past the retina bounces off it, straight back toward the flashlight.
That lets the retina catch the light a second time, helping cats see in dim light. Humans lack this layer, which is why our eyes don't glow like that.