Why does water cause rust?

Hint: Water will cause iron and steel to rust. Dissimilar metals rust faster than single metals due to electrochemical reactions, so steel rusts faster than iron, and joints between dissimilar metals rust very quickly. Saltwater will cause rust faster than water because saltwater may be a better electrical conductor.

Complete answer:
Rust is another name for iron oxide, which occurs when iron or an alloy that contains iron, like steel, is exposed to oxygen and moisture for an extended period of your time. Over time, the oxygen combines with the metal at an atomic level, forming a replacement compound called an oxide and weakening the bonds of the metal itself.
The iron reacts with water and oxygen to make hydrated iron (III) oxide, which we see as rust. Iron and steel rust once they inherit contact with water and oxygen – both are needed for rusting to occur. This prevents the metal below from coming into contact with air (containing oxygen).
Since one of the chemical reactions that cause rust requires the presence of water and therefore the second reaction requires oxygen, rust can only form when both water and oxygen can reach the iron molecules within the nail. Steel rusts also as iron because it's an alloy chiefly composed of iron.

Note:
 Iron oxide, a reddish-brown compound, is generally mentioned as rust. It forms when iron and oxygen react in water or in moisture within the air. The reaction of iron and chloride underwater is additionally mentioned as rust. Certain factors speed up the rusting process, like the salt within the water.