 We'll talk about this in the next video, but before we get to mesons, baryons, and what particles really are fundamental building blocks of matter, we have one other quantum number in conservation law to learn about. This quantum number is called strangeness, and the first hint of its existence appears in 1947 with the discovery of cations in cosmic ray experiments by G.D. Rochester and C.C. Butler. So what was strange about cations? Well, cations and other strange particles were known as v-particles because they produced a characteristic v-shaped decay track within a cloud chamber, like the one shown in this cloud chamber picture here. These v-shapes were consistent with two possibilities. Either they were neutral particles decaying into two particles with opposite charge, or they were charged particles decaying into two particles, one charged and one uncharged. Basically these v-shapes looked like they were characteristic of a decay process. The particles producing these v-tracks turned out to be easy to create in cosmic ray collisions, suggesting that the strong force had a role in their production. But what was odd was that the lifetimes of these particles appeared to be much longer than one would expect for a particle created by the strong force. In other words, another force was responsible for triggering the decay, and it appeared that something was inhibiting or decreasing the decay probability. This means essentially that the lifetimes of these particles were longer than one would expect based on our knowledge of how they were formed. Up until this point, the force involved in creating a particle would be instrumental in triggering its decay. To see a particle that didn't fit this pattern was, well, strange. As a result, these particles were called strange particles. Now the fact that these strange particles were always produced in pairs led to an explanation. A new quantum number in associated conservation law must be playing a role. Physicists called this quantum number S, Strangeness, and in 1953, Murray Gellman at Caltech proposed that the long lifetimes of these particles were because only the weak force could change the particle strangeness, while the strong force and electromagnetic forces could not. So only the weak force could trigger the decay process, in other words. He would go on to win the Nobel Prize for this idea in 1969. Strangeness is adopted as a quantum number, and based on observations, a new conservation law is born. Strangeness is a conserved quantum number for reactions involving the strong force. Protons and neutrons were assigned S equals 0. More exotic hadrons were assigned a range of positive and negative integer values of S, depending on their observed reaction and decay patterns. So to recap, reactions have to conserve charge, linear and angular momentum, energy, leptome number, baryon number, and strangeness, but only for reactions involving the strong force. So as you can see, there are a lot of requirements for each type of reaction. But mess and conservation isn't one of them. As you'll learn in the next video, that's because there's another more fundamental class of particles that we have yet to discover.