cork floats on water because it's density is less then that of water  
iron nail sinks in water because it's density is more then that of water 

i hope it helps............
2 5 2
Density of cork is less than density of water so as the density of cork is less, cork floats in water because surface area of cork is more as compared to its weight. But density of iron nail is more then density of water so it results in sinking of iron nail in water.
1 3 1