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Thomas was placed there by the lab workers at W.I.C.K.E.D. Boys were being sent up once a month in it as part of an experiment. Thomas, however, works for W.I.C.K.E.D. Although his memory was wiped like the other boys, he, and also the girl, knowingly took part in the experiment in order to change the conditions of the test.
How did Thomas manage to get into the elevator at the start of the film?