Red eye reduction
The flash lights up all the blood vessels in the eye which reflect back into the lens. The further away the flash is from the lens the less likely the chances of this strange phenomena occurring. The problem with built-in flash is that you can't control where the flash fires from. To overcome this, many cameras have a red-eye reduction mode, but it is, as the name suggests, only a reducer. It works by firing a pre flash to reduce the size of the pupils, and in doing so reduces the area of red. The redness is still usually apparent, but because the pupil is smaller the effect is less noticeable.