Final answer:
To make the barrel of water weigh 12 pounds, you must remove 8 pounds of water. The riddle is solved by subtracting weight rather than adding, reflecting a common type of math riddle that challenges one's perspective.
Step-by-step explanation:
The question posed appears to be a riddle: A barrel of water weighs 20 pounds. What must you add to it to make it weigh 12 pounds? The riddle's answer is not about adding an actual substance, but rather about removing something. To make the barrel weigh 12 pounds, you would need to remove water. It requires a shift in perspective from adding to subtracting.
Subtracting water from the barrel would achieve the goal. If the barrel weighs 20 pounds and you want it to weigh 12 pounds, you would have to remove 8 pounds of water. This is a simple subtraction problem rather than adding a substance.
To conceptualize, imagine if the water in the barrel were sand. Removing a handful of sand would lighten the weight, similar to removing water to decrease the total weight of the barrel.