What can be completely empty and still weigh a lot?
Answer: An empty truck.
A truck may have no cargo inside at all and still weigh several tons because of the weight of its frame, engine, wheels, fuel system, and other components.
The trick is that “empty” does not mean weightless. It only means the container or vehicle is not carrying anything.
The riddle pushes you to imagine an empty object as something light, but large objects can remain extremely heavy even when there is nothing inside them.
So a perfectly good answer is an empty truck: empty of cargo, but still very heavy.
