Hi there, so my question is as follows. I understand that only the weak interaction can change the flavour of a quark, but why? Idea 1: It's due to the change in flavour also meaning a change in mass, thus a massive exchange particle is needed (gravity is negligible so forget the massive graviton). But then since the exchange particles are themselves virtual, could a photon for example, not simply be 'off mass shell' and produce the same result? Idea 2: It's to do with a change in electric charge (assuming we always deal with U,C,T to S,B,D quarks flavours or vice versa, which i don't know if that is true), thus an electrically charged exchange particle is needed, i.e. W boson (and hence Z boson doesn't cause a change in quark flavour since it's electrically neutral)? Idea 3: The different quark flavours have a different value of weak charge (which is a different quantum number), thus to change this from one value to another requires an exchange particle with weak charge, thus the W bosons are the only gauge bosons that can cause this flavour change (and again, thus the Z boson cannot since it has no weak charge)? I'm sure that someone is about to confuse me with some guage field theory stuff, but I would appreciate an answer, despite how confusing it may seem to an undergrad such as myself. Thank you kindly for any response.