What makes a particle distinguishable from another one? It seems, two different kinds of particles ought to be distinguishable due to different mass or charge or other observable property. But, if particles can change into each other, like neutrinos, it's not clear that we can ever be 100% sure that we can distinguish them. Even heavier particles like neutrons and protons--can we distinguish a neutron from a proton and an electron? Does the concept of distinguishability have a fuzzy boundary, or can it be exactly decided? Is this the same problem as the question of quantum collapse? In a neutron decay scenario, a neutron can be distinguished from a proton and an electron when a measurement is made which collapses the wavefunction. So it seems like objects are distinguishable if we have previously collapsed them into states that ought to look sufficiently different over the course of the experiment. But if we wait too long without making a measurement, we couldn't really distinguish a hydrogen atom from a neutron, could we? Quantum collapse seems to make indistinguishable things into distinguishable. Or maybe, there are no distinguishable things-they only seem so over a certain span of time.