1 kilo of cotton and 1 kilo of iron — Sohan thinks iron is heavy, so 1 kilo of iron will weigh…

Why

Sohan is mixing up two different ideas that usually go together but are not actually the same thing: how DENSE something is (how much matter is packed into a given space) and how much it WEIGHS in total. Iron is far denser than cotton — a small lump of iron packs in much more matter than the same-sized lump of cotton — which is exactly why 'iron feels heavy' as a general impression. But the question already fixed the WEIGHT at 1 kilo for both, before density even comes into it — a kilogram is a kilogram, no matter what it is made of. Test it: picture a huge, room-filling pile of fluffy cotton weighed on a scale until it reads exactly 1 kg, next to a small iron block also weighed until it reads exactly 1 kg — the scale would show the identical number for both, even though the cotton pile looks enormous and the iron block looks tiny. His sister is right: both weigh exactly the same, 1 kilo — density (how packed something is) is not the same thing as total weight once the weight itself is already fixed.