why nails rust

Not all nails rust. My fingernails, for example, show no signs of oxidation.

You think this is another one, don't you, S?

A~

Why nails rust? What kind of a question is this?
Why does water boil? Why does the sun come up in the morning? Why do
the birds crap on my car?
All God willing.

Iron nails rust badly because they are not pure iron. Electrolytic
corrosion takes place between the different constituents, largely iron
and carbon / iron composites. Furthermore, iron oxides expand and
flake off, and inherently offer little resistance against moisture.
Pure iron is far more resistant to rusting but exhibits poor
mechanical strength and is more expensive.

Best

I believe this is what you're looking for.
http://science.howstuffworks.com/question445.htm

Iron and oxygen get together and make iron oxyde (Fe2O3)... if you
want to know WHY that happens... ok, so oxygen isnt stable by itself
(8 electrons, 6 in the last level) in order to become stable it needs
2 more... with iron its a bit more complicated, but its closer to
being stable with 3 electrons less... so then iron atoms transfers
electrons to oxygen atoms, and for each two iron atoms (Fe) you need 3
oxygen atoms... then they ahve oppostie electrical cahrges and they
attract each other... as simple of an answer as i can give.