What logical mistake did this joke make?

I like brother Yun Yun's answer upstairs. Hehe, experts are logically like this. Spiders can hear when they have legs, so they can run. When they have no legs, spiders can't hear, so they don't run. Starting from Hume's puzzle, how to prove the causal relationship between legs and hearing is inevitable ... this is also the basis for us to draw the "objective law". Because there is no solution to the "mystery of fixed mold" and we believe that objective laws exist, we also have reason to explain that spiders' hearing is in their legs. Secondly, Yun Yun's second question is the key, which one is correct and objective. The answer is that we are not spiders. In other words, our knowledge comes from experience and feeling, but whether the phenomenon itself is the essence of things, we have no way of knowing. This spider can also be understood as Kant's thing itself. Logically speaking, it is impossible to explain his mistake. The most important thing is that you put forward another logically correct answer.