a stone is a mean of preposition & want is a public of gills / energy & elements a public of one left requires surprise the public of each preposition ->  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 & teacher is trying to contrast the want of one big break ->  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 slip is a mean of machine -> the student can use balance to know the mass of each rock to compare them