In the 1950s, with development of particle accelerators and studies of cosmic rays, inelastic scattering experiments on protons (and other atomic nuclei) with energies about hundreds of MeVs became affordable. They created some short-lived resonance "particles", but also hyperons and K-mesons with unusually long lifetime. The cause of the latter was found in a new quasi- conserved quantity, named strangeness, that is conserved in all circumstances except for the weak interaction. The strangeness of heavy particles and the μ-lepton were first two signs of what is now known as the second generation of fundamental particles.