$$\require{cancel}$$
What sort of properties shall we be discussing? Perhaps the simplest one is this: The total linear momentum of a system of particles is equal to the total mass times the velocity of the center of mass. This is true, and it may be “obvious” - but it still requires proof. It may be equally “obvious” to some that “the total kinetic energy of a system of particles is equal to $$\frac{1}{2}M\overline{v}^2$$ where $$M$$ is the total mass and $$\overline{v}$$ is the velocity of the center of mass” - but this one, however “obvious”, is not true!