What weighs more

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.