If we describe meat as "tough" it means it is hard and difficult to eat. The mother says that the steak is full of iron (meaning the nutrient iron which exists in food), the daughter understands that the steak has the metal iron in it (which explains why it hard and difficult to eat!)
母親說牛肉中含有大量鐵質（full of iron），但女兒卻以為牛肉含有鐵金屬，所以又硬又難吃（tough）。■漫畫&說明︰英國文化協會
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