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You use a flash to take a photograph because there isn't enough light to take the picture without one. Because the room is fairly dim, the eyes of the subjects are dilated; the pupils are large, to let in lots of light.
The light receptors on the retina of the eye are very well provided with capillaries, the tiny blood vessels that provide blood - and oxygen - to your eyes. (One of the symptoms of hypoxia, or lack of oxygen to the brain, is your sight starting to fade.)
When you take a flash photo, especially if the flash is built into the camera, the light from the flash goes into the eye, and hits the retina. Because the retina is mostly red blood vessels, the red light is reflected while the other colors are absorbed. So the red light bounces off the retina of your subjects' eyes, and back to your camera. Presto! Red eyes!
You can reduce the "red eye" effect by having the flash several inches away from the camera lens. The red will still be reflected, but it won't be reflected straight back at the camera; the reflection will bounce out to someplace away from the camera, and it won't be as noticeable.
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