Select the one substance that is not a rock.

 - Trachyte is a solid. It is not a pure substance.
 - A skull is made by a living thing. It is a solid.
 - Syenite is not made by living things. It is not a pure substance.
Compare the properties of each substance to the properties of rocks. Select the substance whose properties do not match those of rocks.
A skull is made by a living thing. But rocks are not made by living things.
So, a skull is not a rock.
Syenite is a rock.
Trachyte is a rock.