  For each substance-like quantity, a theorem about its conservation or
non-conservation can be formulated. For the electric charge e.g. it reads:
Electric charge can neither be created nor destroyed. Such a statement is short
and easy to understand. For some quantities, however, the proposition about
conservation or non-conservation is usually formulated in an unnecessarily
complicated way. Sometimes the formulation is not generally valid; in other
cases only a consequence of the conservation or non-conservation is pronounced.
A clear and unified formulation could improve the comprehensibility and
simplify teaching.
