Select the one substance that is not a rock.

 - Andesite is a solid. It is not a pure substance.
 - A skull is made by a living thing. It is not a pure substance.
 - Shale is formed in nature. It is not made by living things.
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.
Andesite is a rock.
Shale is a rock.