There is a famous example that electron couldn't absorb the whole incoming photon without emitting another one. Instead of the normal way, I try to prove it simply by argument ( which might be wrong ). There are four constraints in the process, one from energy conservation, three from momentum. If only electron is left after so, there are only three variables (momentum in different directions, energy could be derived from the three variables). Further 'assume' the constraints are all independent, and (Corollary) Given there are n independent constraints with m variables, if m < n, there will be no solution. The photon-all-absorbed configuration doesn't have enough variables, thus impossible to exist. Questions to raise : 1. Is the proof fine? Limit it to at least the case of the photon absorption first. 2. Are the physical laws, especially the energy-momentum conservation, always independent? If not, any example? 3. Is the corollary true? Thank you.