ChatGPT: "One pound of feathers and two pounds of lead weigh the same, which is one pound or 16 ounces. The difference is in their volume, where a pound of feathers takes up more space than two pounds of lead. This is because the density of feathers is much less than that of lead, so even though the weight is the same, the amount of space they occupy is quite different."

As for why it fails, it is likely a bias arising from the question being way more commonly asked in the corpus with equal mass than with distinct mass, increasing attention weights towards an answer expressing equality.

I believe current LLMs lack some common sense at an architectural level. They learn both specialized facts and general deduction in the same weights class: in my mind, they should separate their world model from their instance model.