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 ->  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 ->  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 -> the student can use balance to know the mass of each rock to compare them