Why does an iron nail float on mercury while it sinks in water?

A needle sinks in water whereas a ship made of iron floats on it because :

An iron needle floats on the surface of water due to –

An ice cube is floating in a glass of water. What happens to the water level when the ice melts? It –

A piece of ice is floating in a beaker containing water up to its brim. When whole of the ice melts

While lifting a bucket of water from a well, we feel that the bucket :