eli5 What are phonons? (this is not a typo, I do not mean “photons”)

Question

eli5 What are phonons? (this is not a typo, I do not mean “photons”)

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Answer 1

Systems of vibrations occurring within atoms that make up solids or liquids.

Example: I strike the end of a metal bar with a hammer. That hammer causes atoms at the stuck end to compress slightly, which, in turn, compresses the adjacent, non-compressed atoms. A compression wave (phonon) then travels along the bar, causing the opposite end to vibrate.