Hi, guys. In Povh's book, page 198, he says: "The strong force conserves the strangeness S and so the neutral kaons are in an eigenstate of the strong interaction." I do not see why this must be the case. My atempt to understand it: $$ŜĤ_s |K_0 \rangle = Ĥ_sŜ |K_0 \rangle$$ So $$Ŝ(Ĥ_s |K_0 \rangle) = -Ĥ_s |K_0 \rangle $$ Since the ket ##Ĥ_s |K_0 \rangle## has strangeness -1, it belongs to the eigensubspace of ##Ŝ## with eigenvalue -1. I don't know how one conclude, from this, that $$ Ĥ_s |K_0 \rangle \, \alpha \, |K_0\rangle $$ which is what I want to prove.