What is weightless, can be seen by the naked eye and if you put it in a barrel, it'll make the barrel lighter?
I already know this one, so I won't contribute to it, but I quite like it.
Okay I got one:
I have millions of eyes, yet am shrouded in darkness
I have millions of ears, yet only 4 lobes
I have no muscles, yet I rule two hemispheres
What am I?
My gut says
the human brain