Why does one kilogram of feathers have more mass than one kilogram of iron?

Asked by: David Bishop, New Zealond

It doesn't. Their masses are identical. The only thing that will be different is their volume.

Even if you compressed the feathers until you squeezed all the air out, they would still be seven times the volume of the iron. Fluffed out, as they are on a bird or in a pillow, the volume would be even greater still.

Read more:

• How are birds' feathers waterproofed?
• Can birds fly upside down?

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.