It compares the the nail to how hard it is.
Cora is described as "hard as nails."
This is a simile comparing someone or something to nails. Nails are hard and tough, so this person or thing is being declared to be very hard and tough.
its a simile like Small as an ant a simile is a phrase with like or as
As hard as nails (usually about a person) As hard as diamond As hard as rock
Think about this and you can figure it out. Are nails hard and tough? The kind made of steel certainly are - this simile compares someone's strength and toughness to that of a nail.
It is simile because it have as or ilke
As hard as a brick
One simile I have for you is, ''The rock was as big as an elephant ''.
Billy boy was tough as nails.
it makes your nails grow but it doesn't make it hard
nails
The turtles shell was hard as diamonds.