The Open Door
In a dark room, the black circles in your eyes (pupils) grow very large to let in as much light as possible so you can see. When a camera flash goes off, it happens in a tiny fraction of a second—too fast for your eyes to react and shrink the pupils down.
A Look Inside
The bright flash zooms through your open pupil and hits the back of your eye, called the retina. The retina is backed by a layer called the choroid, which is full of tiny blood vessels. The red you see in photos is literally the light reflecting off the blood inside your own eye!
Mirror in the Back
Human red-eye is mostly just a reflection of blood, but many animals like cats and dogs have a special 'mirror' layer behind their retina called the tapetum lucidum. This helps them see in the dark, but it makes their eyes glow green or yellow in photos instead of red.