A cracked polystyrene man
Who just crumbles and burns

from Radiohead – Fake Plastic Trees Lyrics on Rap Genius

Meaning

Well, actually, polystyrene is thermoplastic, which is in solid (glassy) state at room temperature, but flows if heated above its glass transition temperature (for molding or extrusion), and becomes solid again when cooled

When heated, it doesn’t “crumble & burn”, it “vapors & fumes” (but you get what he’s saying — Thom is criticizing this girl’s smarmy, plastic surgeon boyf)

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