It's probably clear because i understand it perfectly well :)
impudent like always talk back to her teacher and parents.
When the Earth cooled from being hot, all the iron and nickel was concentrated inside the Earth forming the inner two layers, inner solid core and outer liquid core. Then with the passage of time the Earth solidified gradually and the mantle solidified and now at this stage the Earth's crust, the outer most part of the Earth, looks like it's final product (the way it does now).
The underlined word, that, is a pronoun. It is used to describe or identify a thing, person or object. The pronoun stands alone as a noun phrase in the given participant.
