There could be many influences on what is happening and these influences could interact in nonlinear ways such that a unit of change in the input can be associated with quite different changes in the output, sometimes increasing it, sometimes decreasing it, and sometimes having no discernible effect.
Some influences could have a nonlinear effect.
There are definitely no influences that will cause a nonlinear effect.
I don't have anything to say to you.
The man is sleeping
A young woman walks in the field.
The scientist indicated he was good to go.
A woman stand behind the flower stand in the lobby.
A man looks at the camera as he stand outside, while a girl talks on her cellphone.