Ask which slit, lose the wave

What you're seeing

Two narrow slits sit side by side in a barrier; particles are fired at them one at a time, and each one lands at a single, definite spot on the screen behind — there is nothing spread-out or fuzzy about any individual hit. What's strange is the shape the hits make together. Fire enough of them with no marker and the landing spots pile up into stripes: bright bands where hits pile up fast, and dark gaps in between where — no matter how long you run it — nothing ever lands. That's the signature of a wave: the two slits act like two coherent sources, and at the dark gaps their ripples arrive exactly out of step and cancel. A single particle, apparently, has no trouble taking both paths at once, in the sense that matters for where it can land — right up until you check.

Add a which-path marker and fire again: now there's a fact about which slit each particle used — a real, in-principle-readable record, whether or not anyone ever looks at it. The stripes don't fade gradually; they vanish outright. The new pattern is just the plain sum of what each slit would make on its own — the two single-slit humps added together, dark gaps included nowhere. Nothing about the slits or the particles changed. The only thing that changed is that the two routes stopped being alternatives that could interfere and became a fact with an answer.