Answer:
There is an ancient invention. Still used in some parts of the world today. That allows people to see through walls.
What is as big as you are and yet does not weight anything?
Today he is there to trip you up. And he will torture you tomorrow. Yet he is also there to ease the pain, when you are lost in grief and sorrow.
What is never used unless it's in a tight place?
What stays the same size no matter how much they weight?
I always have to work with something in my eye. What am I?