Question

why an iron nail can be sink in water but a wooden block cannot​

Answer 1

because the density of iron is more than that of wood.

Also wood has air space in between.

Answer 2

An iron nail is much more dense than a wooden block. And the density of iron nail is more than that of water while the density of the wooden block is less than that of water. And we know that any thing that is denser than water sinks in it but anything that is less dense than water floats. Thus an iron nail sinks in water while the wooden block doesn't.