As every condensed-matter physicist knows, a phonon is a quantum of sound (or more precisely, of oscillations in a condensed-matter material). Clearly, sound is a wave. But quantum theory teaches us that, in a certain sense, sound is also a particle. Here I want to understand how far the view of a phonon as a particle is justified. To clarify what I mean, here are some more specific questions. - It is often said that phonon is not a particle but a pseudoparticle. What exactly does it mean and how an experiment can determine whether someting is a particle or a pseudoparticle? - Are there experiments showing that phonons are objects localized in space? (Like for photons and electrons.) - Are there double-slit experiments with SINGLE phonons, showing interference of a phonon with itself on the ensemble level, but pointlike-particle nature of a phonon on the individual level? (Like for photons and electrons.) - Does a classical pointlike phonon particle make any physical sense? (Like for electrons and gamma-rays.) - Does first quantization of the classical pointlike phonon particle make sense? (Like for electrons and to certain extent to photons as well.) P.S. Please don't move this thread to the "condensed matter" forum! This topic is about foundations of quantum mechanics and condensed matter physicists usually don't know much about it.