Shine light through two narrow slits and it makes a striped interference pattern — the signature of a wave. The strange part: fire the light one particle at a time, and each lands as a single dot, yet over thousands of shots the same stripes appear. Quantum theory adds probability amplitudes from both slits to predict those stripes. A detector that makes the paths fully distinguishable removes the interference, even if nobody reads it. The pattern rules out a simple picture of independent marbles, but does not by itself prove that no definite trajectory exists.
It is the cleanest demonstration that the quantum world is not just small — it is built on different rules, where the way paths can be distinguished changes the pattern we can observe.
This is the identical fact set, re-told at a different altitude. Switch any time — the reader keeps your place in the idea, not the prose.