What is weightless; can be seen with the naked eye; and, if you put it in a barrel, the barrel will be lighter (weigh less)?

The intriguing concept described is something that truly has no mass of its own. It represents an absence, a void where matter should be. Because it is simply a lack of substance, it inherently possesses no weight, making it an entirely weightless phenomenon. This characteristic is what allows it to defy the typical properties of physical objects we encounter daily.

Despite its lack of physical substance, this phenomenon is readily observable. We can clearly perceive it because it creates a break or an opening in a continuous surface or object, allowing us to see through or into the space it occupies. Our eyes register the difference in light or the lack of material, making its presence, or rather its absence, quite evident to the naked eye.

The most clever part of this puzzle lies in its interaction with a container, such as a barrel. If you were to introduce this concept into a barrel, the barrel would indeed become lighter. This is because creating such a feature in the barrel necessarily involves removing some of the barrel's original material. Whether it is wood, metal, or plastic, the act of making this feature means less of the barrel is present, thus reducing its overall mass and weight. Riddles often play on our literal interpretations of words, and this one masterfully uses the idea of "putting something in" to mean "creating something within" by removing material.