a hole
Helium is a gas that is weightless, odourless, and would make a barrel lighter.
A hole. When you put a hole in the barrel, it reduces the weight of the barrel. But, when you remove the hole, the barrel becomes heavier.
A flashlight
A hole
A Hole
A Balloon pusy
A hole. A hole cannot be seen, weighs nothing, and when added to a barrel, it reduces the weight of the barrel without adding any weight itself.
sand
A hole. A hole cannot be seen, doesn't weigh anything, but when placed in a barrel it reduces the overall weight by displacing some of the volume of the material the barrel can hold.
holes
You can put a hole in the barrel to make it weigh 8 kilograms.
Usually, liquor.