For the process where two neutral pions turn into two charged pions, I noticed that this can occur through both the strong and weak interactions. Through the strong interaction it is a 4-pion vertex, and through the weak interaction it involves a virtual W-particle. If I were asked on a quiz what force is responsible for two neutral pions turning into two charged pions, would the answer be strong force, because the coefficient is proportional to 1/(100 MeV)^2, as opposed to 1/(80 GeV)^2 for the weak force? Similarly, I noticed processes where a nucleon can change into another nucleon can occur through both the strong and weak interactions, with intermediary particles being either a charged pion or W-particle, respectively. So which interaction would I say is responsible for a change in the nucleon?