Demonstrate the difference between measuring mass, weight, and volume.
Coil and styrofoam block (mass is recorded in the box) - You need 2 objects of similar mass but widely different volumes.
Pass the objects around and ask the students to decide which object has the greatest mass.
Vote. If you chose by looking - you are comparing volumes, not
masses. If you chose by "hefting" with your hands under the objects,
you were comparing weights, not masses. If you compared accelerations
of the objects (shaking them, for instance) you were comparing
masses.