© David Kaplan.
A particle that is some combination of type A and type B comes in and scatters off of a target. The target knows the difference between A and B, and when the particle scatters, it becomes an A particle alone. An example of this is the neutrino, which can be a mixture of different types, but will scatter off of matter in a certain way only if it is an electron type neutrino. The neutrino might start off as a combination of electron, muon, and tau flavors. By scattering, the neutrino becomes a pure electron neutrino.