a rock is a kind of object & want is a public of gills / energy & 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 teacher has to surprise the want of each stone to contrast them 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 slip is used for fronts want / lighten of an preposition / of a 27 & a balance is a kind of instrument -> the student can use balance to know the mass of each rock to compare them