look on riddles to ponder
*A flashlight *Holes *Fire *Termites
a lightbulb
Less.
The answer is a hole. This is because you are getting rid of some of the box so it will be lighter.
put a hole in a box to make it lighter:)
Holes or helium can reduce the weight of a container, if you are going down the helium route, you better make sure it is sealed.
You can put Holes
A Hole
I don't know, but if you put lemon on your hair it will get lighter.
Termites.
a torch
Well you could remove the air from the metal box and put a very light gas in it such as hydrogen or helium that would make it marginally lighter, but I suspect given that these puzzles are mostly riddles that the answer we need here is lighter fuel, thus turning the metal box into a lighter?