shining a light on the mineral. soaking the mineral in water. rubbing a penny against the mineral. breaking off a piece of the mineral.
I gave my students this multiple choice question: A student has a mineral sample. He wonders if the sample is the mineral mica. He knows the cleavage pattern of mica is a smooth, flat sheet. The student can best find out the cleavage pattern of the sample by

Only one answer is correct among these 4 choices:
- shining a light on the mineral.
- soaking the mineral in water.
- rubbing a penny against the mineral.
- breaking off a piece of the mineral.

Could you tell me which one is correct?

breaking off a piece of the mineral.