What Weighs More: 1 Kg of Iron Or 1 Kg of Feathers?

Book excerpt: Once, when I was still a child, my father asked me:'Which weighs more: 1 kilo of iron or 1 kilo of feathers?'I remember that sure of myself I answered him quickly: 'Surely 1 kilo of iron weighs more, dad, because metal things, like the armor of medieval knights, are very heavy, while the feathers that decorate birds' wings are very light'.I can't tell you how disappointed I was when I heard him say:'No my son, that's wrong: they weigh the same, because 1 kilo is always 1 kilo!'Maybe because of the evening trick questions from my dad, when I grew up I decided to become a physicist, and to my big surprise I discovered that I was right when I was a child. Indeed: On the Earth, even if only by a little, 1 kilo of iron weighs more than 1 kilo of feathers!Don't worry, the sawdust that I have in my head has not caught on fire! But I hope that I have made you curious and that now you will want to come hunting with me for this strange and heavy mistake.