[digama]

Conservation of mass is a consequence of the fact that matter is not infinitely divisible. The math on this works out too: there is an easy "measure" on three dimensional objects in the real world - just count the atoms. (This is known as a "counting measure" in measure theory.) It is the presence of infinities in the real numbers that causes this measure to fail (you will end up measuring the B-T sets, and regular objects like spheres as well, to have infinite "mass", so that 2*infinity = infinity is no longer as surprising a result).