Wot can you put in a barrel thats visible to human eye but makes it lighter?

What makes a barrel lighter?  What makes Chromosomes visible during prophase?  What can you put in a barrel to make it lighter?  What can you put in a barrel that makes it lighter and you can see?  What can you put into a barrel that you can see that makes the barrel lighter?  What am I I am visible to the eyebut cannot be held by hand I am lighter than a feather but if you put me into an empty barrell I make the barrel lighter?  What is it that you can see it with your eyes cant hold it in your hands and when it goes in the barrel it makes the barrel lighter?  What is weightless can be seen with the naked eye and when put in a barrel will make the barrel weigh less?  It is weightless but you can see it and if you put the barrel of water in it it make the barrel lighter?  What can be seen by the naked eye weighs nothingand when you put it in a barrel it makes it weigh less?  What is weightless can not seen and if you put it in a barrel it will make it the barrel lighter?  What is colorless and weightless but if you put it into a barrel the barrel would become lighter?  It is weightless you can see it and if you put it in a barrel it will make the barrel lighter?  It is weightlessyou can see itand if you put it in a barrel it will make the barrel lighter?  It is weightless you can see it and if you put it in a barrel it will make it lighter?