The quantisation of energy did not arise from an abstract idea, but from a resounding failure: classical physics in 1900 could not reproduce the emission spectrum of a hot body, a curve that any laboratory could measure without difficulty. This section retraces the episode — the definition of the black body and its experimental laws, the ultraviolet catastrophe predicted by classical theory, and Planck’s act of desperation which, by introducing the quantum of energy, opened the door to all of twentieth-century physics.