Rusting of iron articles takes place in the presence of air and water. To test this, take two iron nails. Paint one and leave both of them in the open and in a tray for a few days. After few days, when you go and observe them, you will find that the unpainted nail has rusted while the painted nail is intact.
Now, if you keep the painted nail also for a few months in the open then it will also start rusting.
How to Float a Needle on Water?
Nobody will believe you if you claim that you can float a stitching needle on water. Actually you can. Take a needle and some tissue paper. Take a glass filled with water. Place the needle in a small bit of tissue paper and carefully place it on the water surface. Within a short time, the tissue paper will get soaked with water and can be made to go down with a gentle push with a pencil tip. Now you will find the needle floating in water. This is because of the surface tension of water which tends to collect the molecules on the surface of water into a tiny and elastic film.