Mass(introducing boxes)

mass = volume × density Compare two snapshots, or two may-bes. The conservation of mass connects possible pairs of may-bes. It is a significant constraint on what can happen in the world, even if it does not tell you what will happen, its pretty emphatic about what cannot happen (the may-not-bes).

The mass must remain the same for either snapshot, but the volume and density are compensated quantities.

Teaching examples to use