The optical fiber

There was an optical fiber,
That was eaten by a tiger.
So whenever the tiger would open his mouth,
A little light would come out.

If light wouldn't enter, it wouldnt come out
We know that the exit for light was the tiger's mouth.
The entry for light I leave to your imagination,
Cuz I didn't have enough time for observation.

Oh come on, think of something else, U DIRTY MIND!
It's not necessary that the light entered from behind.