What can you put in a box that makes the box lighter?
Answer: A hole.
The trick is that adding a hole means removing some of the box’s material. Once part of the box is cut away, the box weighs slightly less than it did before.
The wording makes you imagine placing an object inside the box, which would normally make it heavier. But a hole is different because creating one removes material instead of adding weight.
That reversal is what makes the riddle work. The thing you “put” in the box is not a physical object at all—it is an empty space.
