Why do animals' eyes glow in the dark?
Because they have hidden torches.
No.
Wait.
I will explain.
Our eyes have a layer called retina which is made up of photo receptors.
When light enters our eyes, it hits the photo receptors.
Photo receptors detect light, thus making it possible to see.
However, the light which doesn't hit the photo receptors remains undetected.
Poor undetected light.
Now, nocturnal animals like owls, need to see better during the night.
Hence, their eyes have another layer below the retina called tapetum lucidum.
Now, when light doesn't hit the photo receptors, it reaches the tapetum lucidum.
This layer acts like a mirror.
It reflects light back onto the photo receptors, thus giving them a second chance to detect light.
Some of this reflected light also comes out of the animal's eyes.
Thus, making it seem that its eyes are glowing.