The answer is fire. Fire can fall through the air without being harmed in the same way a solid object might be. A flame can be carried downward, moved from place to place, or dropped from a great height and still continue burning if nothing interrupts it. But when fire meets water, it is extinguished.
The brilliance of the riddle lies in how it plays with your expectations. It sounds like a question about strength, when it is really a question about interpretation. The contrast between surviving a fall and dying in water is not about toughness in the usual sense, but about the nature of the thing itself.
That is why classic riddles remain so enjoyable. They are less about obscure knowledge and more about perspective. A simple answer like fire feels satisfying because it was hidden in plain sight, waiting for you to notice the clue that mattered most.
