Answer:
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
who do you think you know but sometimes you don't
What kind of room has no windows or doors?
What can burn the eyes, sting the mouth, yet be consumed?
He has one and a person has two. A citizen has three. And a human being has four. A personality has five. And an inhabitant of earth has six.
Keep doing me to avoid lens dryness