Ateacher describes a mathematical system that can be used to convert the weight of water to the mass of water. what is the teacher most likely describing?

If a teacher wants to represent  a mathematical system which is used to convert the weight of water to the mass of water he surely has to take  idea of acceleration of gravity as a basis when making a description. Teacher will point that weight is mass multiplied by acceleration of gravity which is equal to 9.81 m/s2. And, of course he will mention the definitions of mass ( the amount of matter) and  weight (which  affected by the pull of gravity). These are the most essential points.
Hope that helps.