A double slit experiment which has two stages: first the experimenter marks through which slit each photon went, without disturbing their movement, and demonstrates that the interference pattern is destroyed. This stage shows that it is the existence of the "which-path" information which causes the destruction of the interference pattern, and not a mechanical disturbance (as thought at the time of the creation of quantum theory). The second stage goes by erasing the "which-path" information, and demonstrating that the interference pattern is recovered. It does not matter whether the erasure procedure is done before or after the detection of the photons. So can anyone dispell the mystisism behind this experiment. I thought Richard Feynman explained that "measuring" a quantum process had nothing to do with a councsiess observer. Please try to explain the reasons as simply as possible to a layman.