Question

A cork floats on water surface but an iron nail sinks in it.

Answer 1

1. For a body to float with a little part of it outside the liquid surface,
density of body < density of the liquid
density of cork (0.25 g cm^-3) < density of water (1 g cm^-3).
2. For a body to sink inside the liquid,

density of body > density of the liquid

density of iron nail (7.87 g cm^-3) > density of water (1 g cm^-3)
Hence, the cork will float in water and the iron nail will sink in water.