Today I was reading "Fundamentals of Physics" Resnick, Halliday and Wheeler. There they said that kaon and sigma were always produced together in an equation where pion and proton reacts. Then scientists came up with this new thing called strangeness which needs to be conserved. But I didn't understand why is the strangeness of kaon +1 and sigma -1?
As an addition to the comments of anna v and G. Smith [I can't reply there yet] strangeness is (only) conserved in strong and EM interactions, but not necessarily in weak interactions, e.g. a constituent strange quark decays $s \rightarrow W^- + u$, where you can see there is a strange quark on the LHS and none on the RHS.