Kaon
Kaons are a specific type of meson (mesons are particles made of one quark and one antiquark). What makes kaons unique is that they are made of one up quark or down quark, and one strange quark. (One of the two quarks that make of a kaon must be an antiquark, and the other must be normal matter. For example, a kaon can be made of one up quark and a strange antiquark). The discovery of kaons was significant because it proved the existence of yet another flavour (type) of quark, the strange quark.