Why is this the answer?

The answer is hole because this clever riddle uses a fun word trick to make you think about a hole in a brand new way. A hole is not really a "thing" you can pick up, hold, or carry, and that is what makes this riddle so smart. Let us look closely at the first two clues. The riddle says the answer "cannot be seen" and "weighs nothing." This is absolutely true about a hole! A hole is just an empty space where something solid used to be. For example, the space in the middle of a doughnut is a hole. It is not made of wood or plastic, and it does not take up any solid mass, so you cannot really see the emptiness itself, and you certainly cannot weigh it on a scale. It is just air or the absence of material. Now for the part that can be very tricky: "when put into a barrel, it makes it lighter." When we usually talk about "putting" something into a container, like putting a stack of books into a backpack or water into a bucket, we are *adding* weight to that container. That is the normal meaning of the word "put," but the riddle wants you to think differently. For a hole, the word "put" is the key trick. When you "put a hole" into a barrel, you are actually *creating* the hole. Imagine a wooden barrel. To make a hole in the side of the barrel, you have to cut out a piece of the wood and take it away. That piece of wood is solid, and it has weight. What happens to the barrel when you take the piece of wood out? It becomes lighter! The hole itself is the empty space that is left behind. It has no weight of its own. It is the action of *making* the hole that removes material and makes the entire barrel weigh less. It is an excellent puzzle that makes you stop and think about the simple idea of empty space in a surprising way. This is why a hole fits all the clues perfectly.