Why does a ship made of iron floats while a piece of iron sinks?

Because the ship contains a large amount of air in it. But iron nail doesn't have any space to contain air in it. Air is a lightest thing. That can't sink in water. This method is used in submarines to sink it. There is a tank in the submarine. When they need to sink it they fill the tank with water. To float it they use to fill the tank with air.