A K-meson is a meson that contains one ordinary quark (either an up quark or a down quark) and one strange quark. K-mesons were first discovered in 1947. The shortened name "kaon" came into use in about 1958. Kaons come in two varieties: positively charged and neutral. They are spin 0, weigh about half as much as nucleons, and decay by means of the weak interactions.