Get three empty matchboxes and put a weight in one of them. Lift the weighted box on its own, then put it down and lift all three boxes together. In tests by Isabel Won and her colleagues at Johns Hopkins University, 90 per cent of subjects who tried this said that the weighted box lifted on its own felt heavier than the three boxes lifted together.
“[T]he experience was so striking that subjects often spontaneously and astoundedly commented on its impossibility to the experimenter, and even requested to lift the objects again after the experiment was over,” the authors report. “Anecdotally, those subjects reported that the illusion persisted even during these repeated lifts, including when subjects placed all three boxes on their palm and then suddenly removed the two lighter boxes — distilling the phenomenon into a single impossible ‘moment’ wherein removing weight caused the sensation of adding weight.”
“We suggest that the space of impossible experiences is larger than has been appreciated, extending into a new sense modality. … Impossibility can not only be seen, but also felt.” See the paper for details.