- A rock is a kind of object
- Mass is a property of objects / matter
- Comparing a property of two objects requires knowing the property of each object
- Therefore, comparing the mass of two rocks requires knowing the mass of each rock

- Comparing the mass of two rocks requires knowing the mass of each rock
- Student is trying to compare the mass of two small rocks
- Therefore, the student has to know the mass of each rock to compare them

- The student has to know the mass of each rock to compare them
- A balance is used for measuring mass / weight of an object / of a substance
- A balance is a kind of instrument
- So, the student can use balance to know the mass of each rock to compare them

Therefore, the student can use balance to know the mass of each rock to compare them
[Question]Which tool should a student use to compare the masses of two small rocks?
[Context]- A balance is used for measuring mass / weight of an object / of a substance 
- Student is trying to compare the mass of two small rocks 
- A rock is a kind of object 
- Mass is a property of objects / matter 
- Comparing a property of two objects requires knowing the property of each object 
- A balance is a kind of instrument