somewhere where it gets the flow of water when it rains
It will weigh more because the total weight would be the sum of the person's weight and the weight of the water in the barrel.
Many rural folks put a rain barrel at the bottom of the roof downspouts to collect rain for softwater purposes. My grandmother always did this and used the water to wash her hair and water her plants.
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.
Isopropyl Alcohol
holes
A wooden barrel that holds water is commonly known as a water barrel or a water cask.
You can put a hole in the barrel to make it weigh 8 kilograms.
The barrel cactus stores water in is spongy interior tissues but does not have a water reservoir of water.
Helium.
The barrel is the hallow part of a wave that is moving. The barrel is surrunded by water but still has empty space where the barrel is.
Usually, liquor.
freeze both of the waters and them put them in the barrel. you could still tell them apart since they will be frozen. freeze the water from both jugs and put them in the barrell you could mix the water with (different) coloured jello and lots of sugar so that they harden into two non-mixing lumps in the barral. And not only have you solved the riddle but have a yummy snack afterwards.