Shadow.

My entire existence is controlled by light,

Yet every time a photon comes near me, 

I run. 

The brighter the light, the darker I am,

And when the sun is low in the sky,

I stretch.

Through sturdier silhouettes of trees and buildings,

You stroll down the street, and

I follow.

Across grass and water, over cars and people,

I can only go where you take me.

I'm trapped. 

But when the light is finally gone,

and you lie useless in your bed, 

I'm free.