The answer is a "hole"! Put a hole in a barrel with liquid inside and the liquid will come out of the barrel (and it will be lighter).
The answer is from the riddle, "You can see me. I weigh nothing and if you put me in a barrel I will make the barrel lighter. What am I?"
A riddle is a mystifying, misleading or puzzling question posed as a problem to be solved or guessed. Riddles are of two types: conundrum and enigma.
Here are some similar ones you can try to answer:
Buckets, Barrels, Baskets, Cans. What must you fill with empty hands?
A man takes a barrel that weighs 20 pounds, and then puts something in it. It now weighs less than 20 pounds. What did he put in the barrel?
You have a barrel full of water that weighs 30 pounds. What do you add to it to make it weigh 20 pounds?
Of interest, are you smart enough to assemble brain-teaser furniture? A series of furniture designs by Bulgarian design studio Praktrik (they focus on practical tricks) adapts famously difficult Burr puzzles into tables, chairs and chandeliers that are as handsome as they are challenging to build. Catch more at Wired.