In the double-slit experiment, electrons fired one at a time at a screen with two slits behave as waves — they create an interference pattern, as if each electron passed through both slits at once. But place a detector at the slits to measure which one each electron actually goes through, and the wave behaviour vanishes. The electrons start behaving as particles, going through one slit or the other, with no interference pattern. The act of measurement collapses the wave into a particle. The system behaves differently depending on whether it is being observed. Physicists have been arguing about what this means since the 1920s. The standard interpretation is that quantum systems exist in superposition until measured, and measurement is itself a physical interaction that resolves them. Fine. That is a description, not an explanation. A different framing: the system does not render particle behaviour until particle behaviour is required. Lazy evaluation, in programming terms. The universe seems to optimise for it.