Hello. Can you shed a different light on why mass defects exist please, so that I might finally grasp it intuitively?! I've had a look at these nuclear threads and one about GPE, https://www.physicsforums.com/threads/why-is-there-a-mass-defect-in-the-nucleous.374443/ https://www.physicsforums.com/threads/why-mass-defect.601024/ https://www.physicsforums.com/threads/gravitaional-field-can-store-mass.371845/ ...but they don't exactly answer why the mass of the constituents of a nucleus - whether it be the sun and the earth or an atomic nucleus and its nucleons - is greater when separate than combined. So far, I've gathered the following (in the case of an atomic nucleus - tell me if I'm wrong on any of these points): 1) By mass-energy equivalence, the masses decrease as a result of losing energy, usually by photon emission (I'm guessing this isn't a result of losing matter from the fundamental particle - making it a different particle - but just energy?) 2) The strong nuclear force (SNF) is responsible for this energy change and F = -dV/dr 3) There is always a minimum energy in a bound state that the system will lean towards. 4) It is only the change in energy that is relevant, not the quantity (hence we can have negative energy). 5) Binding energy is another term for nuclear potential energy. 6) Energy is only released in fusion when the binding energy per nucleon of the product is greater than that of the reactants, and vice versa for fission. What my mind keeps playing out that makes me question my sanity here is the following. A neutron and proton are at rest. The system has total energy E = mnc2 + mpc2. An external force then nudges the proton into the neutron for them to bind via the SNF and make a deuteron. The energy dissipated is the kinetic energy of the proton when it reaches the repulsive range of the SNF. Since its rest energy from before the external force acted is not affected, where has its mass gone? Am I missing a trick with my 3rd point above here?