kaon
/ KAY-on /
When physicists first studied cosmic rays in the late 1940s, they kept finding particles that behaved oddly: they were produced very easily but then lived far longer than expected before decaying. The behavior was so puzzling that the new property was simply named strangeness. The particles carrying it included a meson now called the kaon.
A kaon is a meson that contains exactly one strange quark (or one strange antiquark). The positive kaon is an up quark bound to an anti-strange antiquark; there are also negative and two neutral kaons. The strange quark gives the kaon a strangeness of +1 or −1, a label that the strong force conserves but the weak force does not. That mismatch is precisely why kaons are made in pairs by the strong force yet can only decay slowly through the weak force — the original meaning of strange.
The neutral kaon is one of the most important particles in all of physics. A neutral kaon and its antiparticle can quietly turn into each other, and watching them led, in 1964, to the discovery of CP violation — a tiny preference for matter over antimatter in the laws of nature. That preference is a clue to one of the biggest open questions in cosmology: why the universe is made of matter rather than equal parts matter and antimatter.
Positive kaon = up + anti-strange, with strangeness +1. It is made by the strong force together with another strange particle, then decays slowly via the weak force.
The kaon carries one unit of strangeness; its slow decay and matter-antimatter mixing made it a key to CP violation.
Strangeness is not some exotic substance; it is just a count of strange quarks. It seemed mysterious only because the strong force keeps it fixed while the weak force is free to change it.