Which has more mass, a kilogram of feathers or a kilogram of iron?

Both a kilogram of feathers and a kilogram of iron have the same mass, which is one kilogram. Mass is a measure of the amount of matter in an object, and since both objects have a mass of one kilogram, they have the same amount of matter. The difference between the two objects is their density, which is a measure of how much mass is packed into a given volume. Iron is much denser than feathers, which means that a small amount of iron can have the same mass as a large amount of feathers.