Why is one kilogram of iron heavier than one kilogram of cotton?
user
Your question is based on an incorrect premise. One kilogram of iron has the same mass, and consequently the same weight, as one kilogram of cotton.
assistant
oh, why it is that i feel the iron is "heavier"?
user
This is due to an perceptual illusion called the size-weight illusion. Even though 1 kg of iron and 1 kg of feather have the same mass (which is what determines an object's weight), the iron feels heavier because:

1. It is more dense than the feather, so it occupies a smaller volume for the same mass. This makes it seem more compact and heavy relative to its size.

2. It puts more pressure on the skin and hands when lifting, due to its smaller surface area. This cues us to perceive it as heavier.

3. We have an intuitive expectation that larger, bulkier objects should weigh more. Since the iron is more compact, it violates this expectation and feels heavier than its size would suggest.

So in reality, 1 kg of anything weighs the same due to equivalent mass. But various perceptual cues make us perceive the iron as heavier relative to the feather. This is a cognitive bias in how we estimate weight based on other properties.
assistant